My husband and I will be traveling from Rome to Tuscany by train and renting a car for six days. Our VRBO is in the Chianti region (Barberino Val d'Elsa). My first thought was to take the train to Siena and rent from the Hertz location there near the train station. However, I'm wondering if it is easier to take a train to Florence and rent from the Hertz location near the train station. I'm trying to avoid having to go to the Florence airport to rent a car. After Tuscany, we will be taking a train to Milan, so I'm thinking Florence may be the better plan. Any advice is greatly appreciated.
Makes sense, easy
There are many threads addressing this and how to get out of Florence without hitting ZTLs
All drivers must have an IDP
Do your homework regarding driving in Italy
There are multiple rental agencies at the station
If you do, remember that in a short stretch of via della Scala - the stretch between via Rucellai and Piazzale di Porta a Prato - the right lane is reserved to buses. You can recognize it as there is a low yellow lawn divider in the middle - stay on the left. Technically, this is not a ZTL and so it is watched 24/7; it is easy to skip it and drive the wrong lane. - You do not have to necessarily drive via della Scala, but it is close enough that it is likely.
Picking up in Siena makes more sense to me. Easier to get out of and is closer to Barberino.
True but options and open hours will be better in Florence
The easiest way, in my opinion, would be to rent your car in Rome and drive from Rome. Drive the 3 hours from Rome and Barberino VE (nearly all freeway) and you are done.
The second easiest would be to take a train to Chiusi (between Rome and Florence) and the rent a car in Chiusi (Avis-Budget only). Alternatively you could rent in Orvieto (Hertz, Avis-Budget). Then, once in the rental car, drive the rest of the way.
If you must absolutely go to Florence, then rent from Hertz next to the station on Via Alamanni. The warning by Iachera about via della Scala is important because there is really no alternative to via della Scala to get out of that location nowadays.
The route from Hertz to the way to the freeway is here. Use Google Streetview to see how it works. There is a light rail tramway that passes in front of the Hertz office, so you will be driving on the same street (on the tracks).
The Hertz is next to the MH Hotel that you see in this photo (the Hertz office just opened there, so it is not showing in the photo). The Hertz garage is where you see the big P on the blue sign. As you come out you turn right and go straight toward the station (in the background of the photo). Eventually at the station you turn right on Via Santa Caterina and right again onto Via della Scala (in a sort of U turn from your starting point). As Iachera said, once on via della Scala stay on the left lane as on the last portion the right lane is for buses only.
if you must absolutely go to Siena, then it makes sense to go to Siena by bus from Rome Tiburtina station. Numerous bus companies service that route from Rome to Siena, including Flixbus. I think the buses stop at the train station (at the bottom of the hill) before going up to the city center. The rental car offices in Siena are near the station (but not close enough to walk to them so you'll need a taxi)
At the end of this long post I would like to renew my suggestion to pick up your car in Rome, way easier. If you decide to pick up the car at the Rome airport (which however will cost you more in both taxi transfer and airport rental fees), take the A12/SS1 (E80) coastal route via Civitavecchia and Grosseto.