An overnight, definitely, for the city is so very different in the evening when the huge tour groups have left, it's a magical place at night. The first time we ever visited Siena we stayed three nights and reckoned that was just about right for there is a lot to see and experience here.
Siena is a wonderful day trip but the city is particularly lovely at night. I have twice enjoyed a drink on the Campo under a full moon with my husband and it a memorably romantic way to spend an evening. The passegggiata is pleasant and an early morning stroll through one of the Contradi exploring and finding their totems and their fountain etc is fun. So an overnight trip is lovely and there are several central hotels, but you can see the highlights -- climb the Torre Mangia, spend some time in the amazing Cathedral, tour the town hall (you can get a combo ticket with the Torre Mangia). Have lunch on the Campo one of the nicest town squares in Italy on a bus trip from Florence.
overnight one more vote
Thanks to everyone for taking time to answer my question.