Seats on ouigo

For that long of a ride, I would take a regular TGV - shown as InOUI in the reservations system.

To change trains, you have to get off the train and go to the platform where the second train is departing from.

Here is the page from The Man in Seat 61 with seating diagrams for the various TGVs:

https://www.seat61.com/european-train-seating-plans.htm#France

He has an entire page on traveling between Paris and Nice (including photos), and has a section describing the difference between regular TGVs and OuiGos (also about upper vs lower deck). Reviewing that can help you make an informed decision.

https://www.seat61.com/trains-and-routes/paris-to-nice-by-train.htm

If you're looking at dates in the spring, Inoui sales have not opened yet. They will open in January, 24 Jan IIRC, and you can sign up for alerts on SNCF Connect.
On such a long journey, I would only book Ouigo as a last resort cost-saving measure or if all other trains are full.
Changes in Marseille are no big deal: no stairs to negotiate. You get off train #1, go to the main concourse, see from which track your train #2 is leaving, and head there. But there are many direct trains from Nice to Paris so you should not need to change, and you must avoid transferring to a Ouigo train since Ouigo has a 30-min check-in requirement.

I would opt for a direct train only because you have one reserved seat. Many of the trains traveling between Marseille and Nice are local trains with no reserved seats. I've had some uncomfortable experiences trying to find a seat on a full local train on that route and others. There are a few Intercités trains with reserved seats between the two cities, which would be acceptable if a direct train is unavailable.

I echo the advice about paying what it costs to take a TGV. that is a very long trip to be sitting in a rigid plastic seat that is both unpadded and doesn't recline. They are really miserable seats, much like spending 4 or 5 hours on a metro seat. And you don't want to be on a local train in the south as Bets notes with your luggage and no seat.

One update: the Ouigo seats are now padded like any other 2nd class train. I've been in two the last few months.
Are odd seats window seats? I don't have a clue. I was in first class face-to-face solo seats today with my husband, next to a window, # 51 and 52. If you want to be sure you have a window seat, choose a 1st class solo seat. You can't choose your seats in 2nd. Can't guarantee what direction you'll travel, though. My husband rode backwards for 4 hours today.

Thank you to all with your valuable advice
I will only look at tgv trains not ouigo
I have a senor pass so maybe that’s why ouigo keeps showing up ??

Maybe they are the only trains that have posted their schedule. Ouigo, Inouï, local trains and Intercités are all separate companies under the SNCF banner. What's your date? Could be too early.