My wife and I will be going to Sienna from Florence for 3 days. We want to rent a car while in Sienna. Should we rent the car in florence and drive to Sienna or train to Sienna and rent the car ? We will be heading to Sorrento from Sienna. The train goes back thru Florence to get to Sorrento.
I'm not clear when and where you will be driving. If you're just seeing Florence and Siena, no need to rent a car; frequent and easy buses and trains connect the two. Last I looked, the bus was non-stop while the train required a connection and took longer - check. If you rent in Florence, you have to be SURE to avoid the ZTL's; apparently the easiest way to do that is to rent at the airport. I've read about rental agencies that are inside the ZTL's, but don't warn you - be careful. Siena has ZTL's too, but I don't think the rental agencies are so close to them. Other than the ZTL issue, I don't think it matters where you rent; see if you get a better deal at one place or another.
Don, Renting in Florence will allow you to drive S222, the Chianti road, to Siena. Get clear directions out of Florence to avoid the ZTL.
Make sure you have a place to park in Siena only a few hotels inside the walls have parking. Some hotels have deals with the parking lots surronding the city. You can train to Chiusi then on to Naples, you don't have to go back to Florence. Or you could drive to Chiusi or Orvieto to return the car and then train to Naples.