Drive or train to Florence going thru sienna

Siena is not on the main railway/highway itinerary from Rome to Florence. If you drive, it is a detour - not a difficult one, but be sure you understand the difficulties of finding parking in Siena and Florence and accessing most hotels with a car.

If going by train, you should better go all the way to Florence and then backtrack to Siena - either by train or, better, by bus from Florence. The most direct railway itinerary to Siena is shortest than the indirect one trough Florence but slower. Some people also consider bus from Rome (Tiburtina railway station) to Siena.

wynn,

Are you planning to stop in Siena just for the day, or stay there for a few nights? As lachera mentioned, Siena is an easy trip from Florence by bus. Also as mentioned, it can be "challenging" to find parking in Siena and you'd also have to contend with ZTL areas. Taking the train or bus will be less of a problem and likely a more pleasant trip.

For driving in italy, note that each driver registered on the rental form must have the compulsory International Driver's Permit.

I wouldn't rent a car just for that. How long will you be in Florence? If you have time for a day trip from there you could take the bus from Florence to Siena for the day. The bus is better than the train in order to go from city center to city center.

I agree with the others. I wouldn't rent a car if you are staying in Florence. I also agree the better plan would be to take the fast train to Florence, then do a day trip by bus to Siena. Siena doesn't lend itself to dropping in for a couple of hours. If you did a day trip from Florence, I'd take the earliest bus possible out and the latest bus possible back to Florence.

How long will you be in Florence?

The bus from Florence is a very easy way to go there. The bus station is just around the corner from the train station.
When you buy your tickets in Florence they will tell you that you may use the return tickets at any time to return..they are open tickets. There is no reason to obsess over departure or return times. Earlier and later are a choice you may make...
Make sure that you get on the express bus when you return or it will a long day for you.

Thank you all for the suggestions. We will only be in sienna for the day so we’ll make it a day trip from Florence on the bus.