tgv Nice to Marseilles

We did it the day of, and of course, the prices were higher. That being said, booking ahead of time without much leeway in arrival time, you will lose your money if you miss your train. If it's tied to an arrival/departure, we would just buy day of.

I don't think there are any TGVs on that route. It looks as if most departures take about 2 hr. 40 min. I don't believe the trains have reserved seats, so there seems to be no risk in waiting to buy your tickets after you land in Nice--nor is there a price advantage to buying early.

It's a local train that goes along the coast. Buy the tickets when you land.

There are technically both TGV and TER direct trains from Nice to Marseilles. But they are both regular speed trains along this section of track, but with separate tickets. The TER terms are more flexible, I believe.

^ Exactly.
TGV and TER (regional) trains share the same tracks and have similar travel times. TGV have assigned seat reservations and do sell out, TER do not sell out. Both can be bought on the day of travel (except on a sold-out TGV of course).

BIG bonus: Some TER stop at Nice Saint Augustin right next to the airport, whereas TGV never do (you have to go to Nice Ville). So if there's a TER that suits you, it's better.
Trains are unlikely to be crowded in the middle of the day in March.

Last but not least: seat on the left for sea views, and don't fall asleep before you reach Saint-Raphaël: the stretch between Mandelieu and Saint-Raphaël is gorgeous! Toulon to Aubagne is also quite nice.

The TGV trains running from Marseille to Nice can't crank up the speed to "grande vitesse" because of the state of the roadbed (at least that's been the story for many years.)
If this trip starts with a flight, look at flying into Nice which has an airport. If it is international travel, probably a connection would be needed. Even so, it likely would be quicker, cheaper if booked as a single ticket, and less hassle.