I have done this trip dozens of time. If you do not need a car in Marseille nor in Nice, why would you bother with a car, hefty tolls, one-way rent (overpriced in Europe), manual transmission, speed cameras, traffic, the possible parking?
There are several regional trains a day between the two cities, even high-speed (tough riding at the same speed than regional, but with less stops) and the line offers different but scenic views to the see.
The only concerns about the trains are :
- possible strikes issues : the regional line is one the most hit by strikes in France, but seeing no trains is very rare ; solution : book in TGV (high-speed train), the get hit very very less often.
- possible safety issues : the line is at the same time the business, the family, the friend and the beach one, so you will be able to meet every sort of people, including "unfriendly" if you ride on saturday or in the evening ; solution if so : pay a small premium for a 1st class ticket (if available) or book in TGV.
- possible delay issues ; solution : Cf. to traffic jam
