Stay in Sienna or San Gimignano

Without a car: SIENA
With a car: SAN GIMIGNANO

We liked San Gimignano way more than Siena, so I'd choose SG. It's smaller and much more quaint and charming imo.

Thanks!

I'd say stay another night in Florence, and take a day trip. Moving hotels for one night is a hassle, and Siena is an easy 1 hr. bus trip away. San Gimignano is about 25 miles northwest of Siena, and buses also can take you there. With a rental car, you could see both towns in one day, and drive 76 miles from Siena to Orvieto which is just north of Rome.
Orvieto is a great place to spend the night before going into the "big city" on a 70 minute train ride.

I truly loved staying in Siena but we spent 3 nights there. I'm not sure 1 night is worth it in either location. But, if you choose to pack and unpack, I'd pick Siena. If traveling by train, take the bus to the city center at the top (easier on luggage moving) for 1 Euro. There are more things to see and do in Siena. The Duomo has great history and you can climb to the panorama view of the nave that was never finished and have an amazing view of the hills and il Campo. The Piccolini Gallery at the Duomo is beautiful and rivals the art in larger cities. If you are driving, then I'm not sure. Siena is a small gated hill town but I think there is lower parking and you ride an escalator to town. Have fun! We enjoyed both places but San Gminagno is much smaller with 1 main street. Siena definitely has a medieval feel with its steep brick streets on each curve of the mountain.

I really like both places. Siena is bigger and will have more lodging choices as well as things to see and do in the evening. San Gimignano is small but may be ideal for a quiet evening after the tourists leave.