Paris to Marseille should be a ridiculously easy train booking. Three hours or so, direct, done. Then you open the results and suddenly you’re comparing TGV INOUI, OUIGO and Frecciarossa, one fare looks suspiciously cheap, another includes far more, and one of the “Paris” departures may not even leave from central Paris.

For this comparison, I’m sticking to Gare de Lyon to Marseille Saint-Charles, because otherwise the prices become misleading very quickly. TGV INOUI is where I’d start when the departure time matters, simply because it gives you the most choice. Frecciarossa is the one I’d check carefully when one of its four daily departures fits, especially if fares are close and you’d rather not pay extra for every little thing. OUIGO can be a genuinely good buy, but only when the saving survives your luggage and flexibility needs.

And that cheap OUIGO from Marne-la-Vallée-Chessy? Excellent if you’re at Disneyland. Considerably less exciting if you’re waking up in central Paris.

My rule for this route: choose the timetable first, then decide which ticket is worth paying for.

So, which one should you be looking at first?

Have a specific arrival time in Marseille that you really don’t want to mess with? Start with TGV INOUI. It gives you far more choice across the day, which is often more useful than saving a few euros on a train that leaves at the wrong time. I’d still look at the exact fare before paying, because some TGV tickets are much easier to change than others.

Frecciarossa is the one I’d get quite interested in when the timing lines up. There are only four departures a day, so you do have to fit around it a little more, but when the fare is close to the other two, it can be a very good deal because more is already included and the standard ticket is much less restrictive than OUIGO.

OUIGO is different. I’d book it when the saving is properly worth having, especially if I was travelling light and knew the date was fixed. A €19 fare can be brilliant. A fare that only ends up a few euros cheaper after adding the things you need is much less convincing. And this is the one where I’d pay extra attention to the departure station and how early I needed to be there.

Staying around Disneyland Paris or further east along the RER A is one of the rare cases where Marne-la-Vallée-Chessy can be the better option rather than the annoying one. From central Paris, I’d usually filter it out. From that side of the city, I wouldn’t.

And if your dates are still a bit wobbly, I’d probably leave base OUIGO alone and look at Frecciarossa or a TGV fare with decent flexibility instead. Saving money upfront is much less satisfying when changing the ticket later costs almost as much as you saved in the first place.

Before you compare prices, make sure it’s the same trip

This sounds obvious, but Paris–Marseille searches are surprisingly good at mixing together journeys that are not really comparable.

A cheap OUIGO can look like the obvious winner until you notice it leaves from a completely different part of Paris. And once you start comparing €19 with €27 or €35, that matters.

For everything below, I’m comparing Paris Gare de Lyon to Marseille Saint-Charles. That way, the fare difference is about the train you are booking, not an extra journey hidden at one end.

That cheap OUIGO might not be leaving from central Paris

OUIGO also runs from Marne-la-Vallée-Chessy, beside Disneyland Paris, and booking sites can still show that under a broad “Paris” search.

If you are staying at Disneyland, brilliant. It could be the easiest option of the lot.

If you are staying in central Paris, I would look at it very differently. You need to get yourself out to Marne-la-Vallée first, allow enough time for the connection and pay for the local journey too. A very cheap fare can still be good value, but it is no longer the same trip as walking into Gare de Lyon and getting straight on your Marseille train.

So when you see an OUIGO price that looks suspiciously better than everything else, check the station before getting excited about it. That one detail can explain a lot.

At the Marseille end, keep Saint-Charles

For Marseille, I would stick with Saint-Charles when comparing the three trains.

It is the main central station and the common endpoint for TGV INOUI, OUIGO and Frecciarossa. From there, you can take the metro down towards the Vieux-Port or walk into the centre.

The walk is downhill, which sounds easy enough until you add a suitcase, the station steps and a hot afternoon in Marseille. With a small bag, fine. With a large case in July, I would probably be heading for the metro instead.

If another Marseille station appears in the results, I would leave it out of the price comparison unless you specifically want to arrive there. Otherwise you are comparing two slightly different journeys and pretending the only difference is the ticket price.

Staying in France and continuing by regional train? My guide to how TER trains in France actually work covers the bit that changes once you leave the reserved high-speed network.

I really wouldn’t choose between them over seven minutes

Frecciarossa arrived on this route with 3h20 attached to a lot of the launch coverage, but that is not the journey time of every train. Current southbound services run at roughly 3h19 to 3h37, depending on the departure.

TGV INOUI can be a little quicker on some services. I still would not choose my train because one gets to Marseille eight minutes earlier.

What matters much more is whether the departure fits the day you have planned. A 06:38 train and an 11:11 train are completely different choices. A 3h19 journey and a 3h27 journey really are not.

And this is where TGV INOUI has an advantage that is easy to overlook when everyone is comparing speed: there are simply more departures to choose from. I would take the train that fits my day properly over the one that wins by a handful of minutes almost every time.

The cheapest fare is not always the cheapest trip

The trouble with comparing Paris–Marseille fares is that the first number you see is only the starting point. OUIGO has the low-cost reputation, so it is very easy to assume that it will automatically be the cheapest choice, with Frecciarossa somewhere in the middle and TGV INOUI at the expensive end. The fare checks behind this guide did not behave that neatly at all. On some searches OUIGO was comfortably cheaper, on others Frecciarossa matched it, and there were also times when TGV INOUI came surprisingly close to both.

That is why I would never choose the train based on the brand’s usual reputation. I would look at the departures I could realistically take, then compare the price of those specific tickets. A bargain that only exists on a train leaving six hours later is not much of a bargain if it ruins the day you had planned.

OUIGO can stop being cheap once you add the things you need

OUIGO is at its best when the base fare genuinely suits the way you travel. If you have a small bag, your date is fixed and you do not care about choosing a particular seat or adding Wi-Fi, there is no reason to complicate it. In that situation, the low fare can be exactly what it looks like.

The calculation changes once you start adding extras. Seat choice, Wi-Fi, additional luggage and more flexible exchange conditions can all push the final total upwards, while some of those things are already included with Frecciarossa or TGV INOUI depending on the fare. In the fare checks behind this article, adding OUIGO Plus was enough on some searches to wipe out the original saving completely.

I find it more useful to think about the gap than the headline price. If OUIGO still comes out €20 or €30 cheaper after adding what you need, that is a proper saving and probably deserves your attention. If the difference has shrunk to around €5 or €8, I would be much less interested in the fact that one train is technically “low cost” and much more interested in whether the timetable is better and how forgiving the ticket is if plans change.

I would not treat €10 as a strict rule, but it is a useful point to stop and compare properly. Once the difference drops below that, the extra value included elsewhere can start to matter more than the small saving on the screen.

The cheapest departure still has to fit your day

There is one more reason the lowest fares can be misleading: they are often attached to very different departure times. A cheap Frecciarossa in the afternoon is not really competing with a morning TGV if you wanted to be in Marseille by lunch, just as an early train is not automatically better if you had planned a slow morning in Paris.

Booking sites make it very easy to line up the lowest fare from each operator and compare them as though they were interchangeable. I would do the opposite. First narrow the search to the trains you would genuinely be happy to take, then compare the final fares within that window.

That small change makes the decision much more realistic. Paying a few euros more for a departure that gives you most of an extra day in Marseille can easily be better value than saving a little on a train that leaves at a completely useless time.

The cheapest ticket can get expensive if your plans move

This is where the three trains separate much more clearly than they do on speed or comfort. A low fare is easy to love when the date is fixed; it is a lot less appealing when there is even a decent chance you might need to travel a day later.

The important thing is to look past the word “flexible” and check what that really means for the ticket in front of you. An exchange fee, the difference to the new fare and the amount you lose if you cancel are three separate things, and they can add up very differently depending on which train you book.

Frecciarossa is the easiest one to understand

Frecciarossa’s standard French fare is refreshingly straightforward compared with the other two. Under the current Serenità conditions, you can change the journey up until departure without paying an exchange fee, although you still pay any difference if the new train costs more. If you cancel instead, 20% of the fare is retained.

That does not make it fully refundable, but it does give you a fairly clear idea of the most you are risking before you book.

Say the fare is €50 and there is a real chance your plans will change. Losing roughly €10 on a cancellation can be much easier to swallow than discovering that the cheaper alternative cannot be refunded at all. It is one of the reasons Frecciarossa can become more attractive when the price difference between the trains is small.

With TGV INOUI, the fare name matters more than the logo

TGV INOUI is more complicated because the rules depend on the fare you choose. Some eligible tickets can be changed or cancelled without a fee until several days before travel, then become more restrictive closer to departure, while cheaper fares may have much tighter conditions from the start.

That means I would never see “TGV INOUI” on the booking screen and assume I knew how flexible the ticket was. I would open the fare conditions and check the exact exchange and cancellation wording before paying, particularly if the trip is still a few months away or tied to something else that could move.

There is another small trap here: after an exchange, the refund conditions can change. A ticket that started reasonably flexible may not keep exactly the same cancellation rights once it has been moved, so this is one of those moments where the boring little fare-condition link deserves thirty seconds of your attention.

OUIGO Flex does not make the ticket refundable

OUIGO is the one I would be most careful with if my dates were uncertain.

The base ticket is non-refundable by choice, and a voluntary exchange currently comes with a fee unless you have bought OUIGOFLEX. Flex removes that exchange fee, but it does not remove any increase in fare and it does not turn the ticket into a normal refundable fare.

That distinction matters. If your €30 OUIGO is moved to a train selling for €55, Flex means you avoid the exchange charge, not the extra €25. And if you decide not to travel at all, Flex is not a free cancellation option.

OUIGOSWAP can sound like a refund route, but it is really a resale system. If another traveller buys your eligible ticket, you receive a voucher worth 80% of the original ticket price. If nobody buys it, there is no guaranteed payout, and the money does not come back to you as cash.

That is why I would be quite happy booking OUIGO for a trip I knew I was taking, especially when the saving was substantial, but much less enthusiastic if the date was still moving around. In that situation, Frecciarossa or a genuinely flexible TGV fare can be easier to live with even if the starting price is a little higher.

And whichever one you choose, read the conditions attached to that exact fare before you pay. SNCF’s general product pages and its detailed fare pages have not always shown the same wording, so the ticket conditions at checkout matter more than a broad promise that something is “flexible.”

If your plans might move, this is where the fares really separate

A cheap fare is easy to love when you know exactly when you are travelling. It becomes a very different purchase when there is even a reasonable chance the date could shift.

This is the point where I would stop comparing the headline prices and ask a much more useful question: how much could this ticket cost me if I need to change it later?

There are three different things hiding behind the word “flexible”: the fee for changing the ticket, any extra you have to pay if the new train costs more, and what you lose if you cancel altogether. Those are not the same thing, and on Paris–Marseille they can make two similarly priced tickets behave very differently.

Frecciarossa is the easiest one to live with

Frecciarossa’s standard Serenità fare is probably the simplest of the three to understand. You can change the journey up until departure without paying an exchange fee, although you still cover any increase in the new fare. If you decide not to travel, you can cancel before departure and 20% of the ticket price is kept.

That becomes quite appealing when the fares are close.

Imagine Frecciarossa is €49 and OUIGO is somewhere between €39 and €49, but you are not completely sure which day you will return. Cancelling the Frecciarossa would cost you roughly €10. With a standard OUIGO ticket, changing your mind about travelling altogether is a much more expensive problem because the ticket is not refundable by choice.

I would not pay a huge premium just for the possibility of cancelling, but if Frecciarossa is only a few euros more, that extra room to change plans starts to feel like money well spent.

With TGV INOUI, read the fare rather than trusting the train name

TGV INOUI can be every bit as sensible for an uncertain trip, but you need to pay closer attention at checkout because the flexibility depends on the fare you choose.

Some tickets give you plenty of room to change or cancel, while cheaper fares can be much less forgiving. The conditions can also tighten as the departure gets closer, so I would never see “TGV INOUI” in the search results and assume I already knew what would happen if my plans changed.

This is one of the rare moments when I would genuinely click into the fare conditions before paying. SNCF’s dedicated exchange and refund information is more useful than the short product summaries, but the wording attached to your specific ticket is the bit I would trust.

Also check what happens after you make a change. Some TGV tickets can become less refundable once they have already been exchanged, which is exactly the sort of detail that only becomes interesting when you suddenly need it.

OUIGO Flex helps, but it still doesn’t give you a refundable ticket

OUIGO is much easier to recommend when your dates are properly fixed.

The standard ticket is non-refundable by choice, and OUIGOFLEX does not change that. What Flex does is remove the exchange fee. If the train you move to costs more, you still pay the difference.

So if you bought a €30 OUIGO and the only suitable train on your new date is €55, Flex saves you the exchange charge, but you are still paying another €25. And if you decide not to travel at all, Flex does not suddenly turn into a cancellation refund.

You may also come across OUIGOSWAP, which sounds more generous than it is. Your ticket can be offered for resale, and if another traveller buys it, you receive a voucher worth 80% of the original fare. If nobody buys it, there is no guaranteed refund waiting for you. It is a useful backup, but I would not book an uncertain trip assuming Swap will rescue the money later.

For a weekend I knew was happening, I would have no problem choosing OUIGO when the saving was good enough. If the dates were still moving around, I would be far more tempted by Frecciarossa or a TGV fare whose change and cancellation rules I had checked properly.

And one small booking decision can make all of this less annoying later: exchanges and refunds are handled by the seller that issued the ticket. When the direct fare and the aggregator fare are the same, I would usually book with the operator simply so there is one less company involved if something needs changing.

Three hours is long enough to notice the differences

Once I had narrowed the search down to trains that suited my day and my budget, this is where I would finally start comparing the onboard experience. Paris to Marseille is not such a long journey that I would rearrange everything for the fanciest train, but three hours is still plenty of time to care about where I’m sitting, whether my phone is charging and whether I can get a coffee without having to plan it before I board.

I would also be wary of anyone confidently declaring one of these trains “the most comfortable”. Seats vary between train sets and people have very different opinions about firmness, legroom and what counts as comfortable for three hours. The differences I find much more useful are the ones you can know before booking: can you choose the seat, is Wi-Fi included, is there power, and can you buy something to eat?

Frecciarossa gives you quite a lot before you upgrade anything

Frecciarossa has the strongest Standard-class package of the three on paper. You can choose your seat, Wi-Fi is included, and Trenitalia says every seat has both power and USB. There is a bar car too, and you can choose between Silenzio and Allegro areas depending on whether you would prefer a quieter carriage or one where conversation is less likely to earn you a look from across the aisle.

That combination becomes quite persuasive when Frecciarossa and TGV INOUI are sitting close together in price. You are getting several little conveniences in the basic fare rather than deciding at checkout which ones matter enough to pay for.

TGV INOUI second class is less generous with seat choice, because you normally select a preference rather than opening a full seat map and choosing the exact place yourself. Wi-Fi is included, power is available on most trains and there is a buffet on board. For me, that is more than enough for Paris–Marseille, particularly if the TGV leaves at a much better time. I would not take an awkward Frecciarossa just because its Standard cabin wins the specification comparison.

OUIGO is more stripped back. Your seat is assigned, while choosing a particular seat and adding Wi-Fi generally costs extra or comes through a bundle such as Plus. Power is also something I would check on the seat or offer being booked rather than assuming it will be there.

None of that makes OUIGO uncomfortable by default. For three hours, I would quite happily sit in an assigned seat with a book and headphones if the ticket had saved me a decent amount of money. I would just go into it knowing I had bought the simpler version of the journey.

I’d always check the upgrade price before dismissing first class

I would not automatically book first class or Business for a journey this length, but I would always have a quick look at the price difference.

TGV INOUI first class gives you more space, wider seating and proper seat-map selection. Frecciarossa Business also adds more room, with Trenitalia listing 69 cm seats, a wider aisle, a divider and a welcome snack. If the upgrade happened to be modest on the departure I already wanted, that could be money I was quite happy to spend, especially on a busy travel day when I wanted a quieter few hours.

If the difference was large, though, I would stay in Standard or second class. Three hours is not long enough for me to justify paying a serious premium simply because one cabin has a nicer label.

There is also a useful reality check in the limited first-hand evidence available. One reviewer travelling Frecciarossa Business between Lyon and Paris described the ride as quiet and smooth but found the seat firm, and the included snack was hardly the highlight of the journey. That does not tell us how everyone will feel about Frecciarossa, but it is a good reminder that Business does not automatically equal a better seat for every person.

The OUIGO food situation is the one I’d plan around

The biggest onboard difference for me would probably be food, simply because it is the easiest one to forget about until the train has left.

TGV INOUI has a buffet and Frecciarossa has a bar car. OUIGO has neither. On a three-hour journey, that is hardly a reason to avoid OUIGO, but I would buy my coffee, water and something to eat before boarding rather than discovering halfway to Marseille that my only snack is whatever happened to be in my bag.

That is probably the best way to think about comfort on this route. I would never pick a less convenient departure just because one train has a bar car, and I would not pay a big premium for a wider seat or onboard Wi-Fi. But if two trains leave at roughly the same time and cost about the same, those small differences start to matter a lot more over three hours.

When the journey goes wrong, the timetable suddenly matters a lot more

On a normal travel day, the difference between having four departures or many more can feel fairly academic. You pick the train you want and get on with it. The difference becomes much more real when your train is cancelled, heavily delayed or you are trying to protect an onward connection from Marseille.

I could not find comparable Paris–Marseille punctuality data that would justify calling TGV INOUI, OUIGO or Frecciarossa the most reliable. So I would not choose between them on that basis. What you can compare is how much room the timetable gives you to recover when the day starts going wrong.

TGV gives you more ways to salvage the day

This is where TGV INOUI’s bigger timetable becomes genuinely useful. Frecciarossa currently has four departures a day in each direction, while TGV INOUI gives you far more choice across the route. If one train disappears from the board, that difference can matter much more than anything we have said about Wi-Fi, seat choice or a few minutes of journey time.

It still does not mean you can simply move yourself onto the next TGV. There has to be availability, and whatever alternative is offered depends on the disruption and the ticket you hold. But having several later services on the same network at least gives the operator more possible ways to keep you moving.

If I had an onward train from Marseille Saint-Charles, a flight later that day or anything else with a hard deadline, this is one of the reasons I would lean towards TGV INOUI and still leave more time than I thought I needed. I would rather arrive early and have a coffee than spend the last hour of the journey watching a connection disappear one minute at a time.

In case Marseille is only the start of the trip, these are the small towns near Marseille I’d genuinely get on a train for.

The next train on the board may not accept your ticket

This is probably the most important thing to understand on a bad travel day: TGV INOUI, OUIGO and Frecciarossa are not interchangeable simply because they are all going to Marseille.

If your Frecciarossa is cancelled and a TGV leaves twenty minutes later, do not assume you can board it with the same ticket. The reverse is true too. Any move onto another service has to be authorised as part of the disruption arrangements.

OUIGO is restrictive in the same way. Under its current cancellation rules, the replacement option can be another OUIGO within the permitted window and subject to seats being available. That is very different from having access to every Paris–Marseille train leaving Gare de Lyon that afternoon.

It is a small distinction on a normal day and a very big one when you are standing under the departure board trying to work out how you are getting south.

Compensation helps later. It does not save the connection

The compensation rules are different too. TGV INOUI’s G30 starts from 30 minutes, with 25% compensation for a delay of 30–59 minutes. Frecciarossa France also starts compensating at 30–59 minutes, currently at 25% in the form of a digital voucher. OUIGO normally starts at 60 minutes, when the standard compensation threshold kicks in.

Those differences are useful to know, but I would not choose my train because one operator starts paying compensation thirty minutes earlier. If I have missed something important in Marseille, a voucher is not really the part of the story I care about most.

What matters more is how easily the journey can be put back together. That is why TGV INOUI gets the edge from me here: not because there is evidence that it runs more punctually, but because the larger timetable gives you more possible ways out when one departure fails.

And if you do need a refund or exchange after disruption, remember that the seller who issued the ticket handles the process. When the direct fare and the aggregator fare are the same, booking with the operator keeps that chain a little shorter, which becomes much more appealing once something has gone wrong.

If French train names already feel like enough to decode, Spain has its own version of the same problem. I’ve broken down how Renfe trains in Spain work too, including the different train types and what you’re really booking.

When I’d choose OUIGO, TGV INOUI or Frecciarossa

By now, the differences between the three trains are pretty clear. The more useful question is what happens when those differences collide with a real trip, because a big suitcase, an uncertain date or an awkward departure time can change the answer completely.

You’re travelling light and OUIGO is saving you €20 or more

I’d take OUIGO from Gare de Lyon and be quite happy with it.

This is the version of the trip where paying more starts to feel unnecessary. If the date is fixed, the bag fits and the assigned seat does not bother you, a proper saving is more useful than paying extra for things you may barely notice over three hours.

A few euros would not sway me. Twenty or more probably would.

Heading back north through Lyon later in the trip? These are the small towns near Lyon I’d actually get on a train for.

Frecciarossa is within about €10 and the timing works

I’d lean towards Frecciarossa.

At that point the gap is small enough that the included seat choice, Wi-Fi, charging and more forgiving standard fare start to feel more valuable than squeezing the last few euros out of the booking.

The important bit is that the departure has to suit you. I would not hang around Paris for half a day just because Frecciarossa happens to look better on paper. If the timing is right, though, this is probably the easiest one to justify when prices are close.

You need to be in Marseille by a certain time

I’d look at TGV INOUI first.

This is where having more departures matters most. If you need to arrive in the morning, make an onward train or simply want most of the day in Marseille, I would choose the train that fits that plan rather than the one with the nicest Standard-class package.

A better timetable beats a small fare saving very quickly on this route.

Your suitcase is big, deep and probably overpacked

I’d measure it before booking anything, and TGV INOUI would probably be my first check.

Frecciarossa may include two cases, but that 31 cm depth limit is the one that can catch out a larger hard-shell suitcase. OUIGO can still work with the correct add-on, but once you need to start paying for extra baggage, the price advantage needs another look.

If I had already hauled a heavy suitcase across Paris, I would much rather know it fits than spend boarding wondering whether the wheels count in the measurement.

There’s a decent chance your plans will change

I would avoid base OUIGO and start with Frecciarossa or a TGV fare that is genuinely flexible.

Frecciarossa would probably be the first one I checked because its standard conditions are easier to understand. TGV can be just as good, but only once you have looked at the exact fare in front of you.

This is one of those situations where the cheapest ticket can be the one I trust least. If the return date is still moving around, I would rather pay a little more now than discover later that changing it wipes out the saving.

You’re staying at Disneyland Paris

This is the one time I would stop treating Marne-la-Vallée-Chessy as the awkward OUIGO option.

If that is where you are already staying, leaving from Marne-la-Vallée may be far more convenient than travelling back towards central Paris just so you can depart from Gare de Lyon. In that case, compare the OUIGO fare properly and judge it on the trip you are really making.

That is a completely different calculation from someone starting their morning in central Paris, and one of the few cases where the “outlying” station can genuinely be the better choice.

If this trip is part of a bigger France itinerary, I’ve also rounded up the French towns I’d choose over another Paris stay.

Five things I’d check before I paid for any of them

Once I had narrowed it down to two or three sensible departures, I would stop comparing every tiny difference and run through these five checks. They are the ones most likely to save you from the slightly maddening moment when you realise, five minutes after booking, that the cheap train leaves from the wrong station or your suitcase was never included.

Make sure the stations are the ones you think they are. For most trips, I’d want Gare de Lyon to Marseille Saint-Charles. If Marne-la-Vallée suddenly appears with a tempting fare, I’d only keep it in the running if I was already staying around Disneyland or east of Paris. Otherwise, that cheap ticket has just given you another journey to organise before breakfast.
Pick the departure time before you obsess over the journey time. I would happily take a train that is eight minutes slower if it means arriving in Marseille at 11:00 instead of halfway through the afternoon. On this route, the difference between 3h15 and 3h25 matters far less than whether the train fits the day you were planning in the first place.
Add the extras you would really buy. Not every upgrade the booking screen throws at you, just the things you know you need. A bigger suitcase, a chosen seat or more flexibility can be enough to turn a very cheap OUIGO into a fairly ordinary deal, so I’d compare again once the fare looks like the ticket I would genuinely travel on.
Click into the fare conditions, even though nobody enjoys doing it. I want to know what happens if I wake up a week later and realise the date needs to move. Can I change it? What would I pay? What do I lose if I cancel? Thirty seconds here is much better than trying to decode the rules when the trip has already gone sideways.
Check the boarding time, then book direct when there is no real advantage to doing otherwise. OUIGO needs a bit more margin than the others, so this is not the train I would arrive for with six minutes to spare and misplaced confidence. And if SNCF or Trenitalia is selling the exact same ticket at the same price as an aggregator, I would usually buy from the operator. One less company to involve later is a small thing until you suddenly need to change something.

Those five checks are enough. If you are still comparing socket locations and seat maps after these five checks, you may have officially spent longer choosing the train than the train journey itself :)

A few things you might still be wondering

Are TGV INOUI and OUIGO basically the same train company?

Yes. Both belong to SNCF, but they are sold very differently. TGV INOUI is the fuller-service product, while OUIGO is the lower-cost version with more restrictions around baggage, boarding and ticket changes.

Frecciarossa is the separate competitor here and is run in France by Trenitalia France. So when you are comparing TGV INOUI and OUIGO, you are really comparing two SNCF products. Frecciarossa is the genuinely different operator.

How early would I get to Gare de Lyon?

For OUIGO, I’d give myself around 30 minutes. Its boarding process is stricter, and I really would not make this the train I arrive for at the last possible minute.

For TGV INOUI or Frecciarossa, around 20 minutes is a sensible margin if you know Gare de Lyon already. I would add more time if it is your first visit, you have a lot of luggage or you need assistance.

There are official minimum cut-offs, but I would not plan my morning around them. Gare de Lyon is large enough that a relaxed ten extra minutes feels much nicer than jogging through the station while checking the departure board every twelve seconds.

Can I use Interrail or Eurail on Frecciarossa between Paris and Marseille?

Yes. Interrail and Eurail pass holders can reserve Frecciarossa Paris–Marseille, but you still need the required reservation for that specific train.

I would check the live reservation information when you are ready to book, because quotas and booking channels can change. It is one of those details I would never assume from an old forum thread.

Which one would I choose with children?

I would probably start with TGV INOUI or Frecciarossa, then compare the total family fare rather than getting distracted by one cheap adult ticket.

With children, I would care more about being able to sit together, how much luggage we were bringing, how awkward it would be if plans changed and whether the departure time was convenient. Frecciarossa becomes appealing because seat selection is included and the standard fare is fairly forgiving, while TGV INOUI gives you far more choice over when to travel.

OUIGO can absolutely still be the better deal for a family, but I would only call it cheaper after adding any seating and baggage extras you would genuinely need.

Paris to Marseille by train: the cheapest fare can be misleading