sienna or san gimigiano

I would definitely go with Siena. There are no bad choices here. Siena has a lot of sites to offer a traveler. Easy bus connections between Florence and Siena. You can day trip or overnight if you chose.

Month of the year makes a big difference in Tuscany, especially the crowded summertime. The Siena Cathedral floor is not exposed during the high season. You didn't say if you have a car. Both towns can be annoyingly crowded.

I'd also pick Siena, because it has more than you can do in a day. But in high season, I would advance-purchase the correct town-pass that includes the Cathedral and the things you want to do. That's because you will otherwise not get into the Cathedral, and may spend hours in line for things you do want. Check the Palio schedule so you can avoid it, but maybe get to see the setups for the neighborhood parties.

will be in florence the first week of october, no car

We are in Siena now. We walked right up and purchased our passes today with no line and no wait for the cathedral. I guess we are lucky. We did get to pass through a Contrada party (Onda) last evening (with permission). It was awesome! Palio preparations are in full force.

Both Siena and San Gimignano were both really nice placed to visit. We stayed 3 nights in Siena and simply loved it last year. In fact, we are going back again this fall. Siena is one of my favorite places to visit, the Duomo, gardens, and people are simply lovely.

Siena! While San Giminiagno is a pleasant village, it's a bit too touristy now for my taste. The Campo in Siena is like nothing else! Plus their cathedral and bell tower are worth seeing. Have fun, good luck!

Definitely Siena! Great to walk around and explore. The Campo is wonderful for people-watching. San Gimignano is nice, but I think there's more to do in Siena.

Why not both?

Sienna is much bigger,San Gimigiano would fit into a couple of Sienna neighborhoods but we have enjoyed both and they are crowded with tourists because of what they have to offer,after all what are we.
Mike

Sienna. And for me - its not even close. Their duomo is spectacular (I'm not religious - its just architecturally amazing.) And you can't really beat sitting in the Campo having lunch.

Siena for sure. It is much larger and has so much more to do. An easy bus ride to get there. San G is very small, crowded, and touristy.