Best to rent a car in Sienna or Florence?

Rent the car in Florence and drive.
I would return the car in Rome (2.5 hour drive) as soon as you arrive (you don't need a car while in Rome).

If you don't want to drive the 2.5 hours to Rome, and rent from Avis-Budget you can return it to Chiusi (not far from Siena, then take the train to Rome (Chiusi-Chianciano station is walking distance from Avis in Chiusi).
Hertz has an office in Orvieto, about half way to Rome (near the Orvieto train station), then you can take a train.
The above are small franchise offices which are open only Mon-Friday and Sat morning (and also observe a 3 hour lunch break during the week).

Going to Rome from Siena is cumbersome by train (must go back to Florence). However you can take buses (like Flixbus)

Check rental rates on www.autoeurope.com or www.kemwel.com (same company now after they merged, but they retained both websites). AutoEurope does not work with the Orvieto Hertz office anymore, I think because of poor customer service at that location.

Thank you. This is very helpful.

Welcome to the forum!
Be sure to do your homework on driving in Italy as far as ZTLs, speeding cameras, parking etc

All drivers must have an IDP International Driving Permit obtained at your local AAA before you leave the states about $25

Make sure your hotel in SIENA ( note spelling) has easy access secure parking, especially if you plan to come and go on day trips

This thread includes Roberto’s very helpful directions for driving safely out of Florence:

https://community.ricksteves.com/travel-forum/italy/dropping-rental-car-off-in-florence

Have fun!

I’ve just been through this whole process and something to consider is one way fees. These can be upwards of 100-150 euros so not cheap! Also pick up and drop off times. We looked into renting in Siena but found the rental office to be closed due to a festival holiday. Auto Europe is great for research but I found booking directly with the rental agencies to be cheaper, plus they provide free cancellation without an additional cost.

plus they provide free cancellation without an additional cost.

AutoEurope has free cancellation with no addition cost as well. We have often booked with AutoEurope, then watched the rates when we see a lower rate we cancel and rebook no problem.
We like them for their excellent customer service and 800 number 24/7 availability if needed.

You might also consider taking a train from Florence to Chiusi, renting a car there, then returning it there before taking a train to Rome in order to avoid the fee of picking up and dropping off in different locations.

I've also rented through AutoEurope and found them quite easy to work with. I monitored prices after I made a reservation, then made a quick call to get a refund when they dropped. I also had free cancelation with them.

Roberto has the very best advice for navigating rental car issues in Florence., and has the very best directions for getting out of Florence. We just returned from Italy, renting a car in Florence and returning it to the Rome airport. We used Autoeurope once again, and again have been very happy with their customer service. Because of a change in plans, I changed our reservation (through Autoeurope) to a car from Europcar, to be picked up relatively near the train station, on Borgo Ognissanti. After taking a taxi to their office, we found out that their cars are no longer there (maybe until 2026?), but are back at the parking garage at the SMN train station. Good thing we packlight…so schlepping back to the train station and locating a tiny (picture valet stand) area where we first ignored, were basically tossed car keys, told it was in a numbered stall, and given a ticket that didn’t work in the garage exit (picture cars piled up behind us). Our car had some serious damage (be sure to always check, photo and date stamp before you leave with the car), but our paperwork showed no damage. So we’ll be contacting Autoeurope so that they are aware of the location situation….travel can have hiccups, and we usually laugh and consider the experience, but this was just not ok and caused a 2+ hour delay in reaching our destination near Pienza. So beware Europcar Downtown Florence office at this point. To be fair, the Rome airport location was great, the employee just shaking his head and said that the Florence location was more of a franchise, and not operating to the Europcar standards in Rome, Milan, etc.

Most offices outside the airport locations and some major train station locations are independent franchises (not only Europcar, but also other companies, like Hertz in Orvieto or Avis-Budget in Chiusi). Some of those franchises have good customer service, some are border criminal. Make sure you get NO DEDUCTIBLE insurance coverage. If you use AutoEurope or Kemwel (same company) you can click on that option under the coverage menu. If you don’t select the No Deductible option, make sure your credit card will reimburse you for any deductible in Italy (not all credit cards do so in Italy). Somehow those independent franchises always manage to find scratches (often not even caused by you) and charge you hundreds of euro for the deductible (which is generally up to 1,000€) on the repair. It’s extra profit for them, because I can assure they will not bother to repair that scratch, they will simply use it to play the same gimmick on the next customer.