It’s up to you. The car rental returns in the city center are walking distance from the station. If you return at the airport car rental Center on Via Palagio degli Spini, besides being on the opposite side of the city compared to where you come from, you have to make your way to the station either by taxi (about 25€) or, more cheaply for less than 2€, by light rail (the light rail leaves from the airport terminal so first you need to take the free car rental shuttle from the car rental center to the terminal). The shuttle bus takes 5 min to the terminal, the light rail line T2 takes about 20 min from airport to SMN station.
If you are comfortable driving in the city streets of a big city, you can return downtown, if you are comfortable only on freeway and extra urban roads, and not in city streets, then the airport might be better in spite of the extra time and the extra expense.
All the info on how to return the car at any location (including how to avoid the ZTL cameras, are in the post thread below.
https://community.ricksteves.com/travel-forum/italy/dropping-rental-car-off-in-florence