Tuscany

Where will you be flying out of? What are your interests? Will you have a car?

I would have to say Venice, even though it will be cool, it is still Venice, and with fewer tourists it is even more pleasant. Has anyone been to Italy before?
(Hello from lower Michigan:))

Venice would be a good choice. You wouldn't want a car there, so you could drop it when you get there and train back to Rome. Another place you might consider is Verona. There is a lot to see in that area and you could do a day trip to Venice. If you'd rather go in the other direction, how about the Amalfi Coast? You could make time for Pompeii.

Venice is good. October, generally you get good weather. We were there in Oct 2008 and it was hot! Also consider the most beautiful place on earth, Lago di Como and Bellagio.

I vote for Venice also, booking an apartment since it's such a large group. I've been there the last two Octobers for a week or more each time, and will be back again in Oct of this year. Weather has been good each time, generally shifting from quite warm midday (cool morning/evening) to chilly at some time during my stay... never really cold (and my definition of cold probably varies markedly from yours).

Venice is great. If you can't do that, Florence has some amazing renaissance sights.

If you are in Siena, make sure that you see Cortona and some of the other Hill Towns as recommended by Rick Steves. In fact, I've used his books and videos as the basis for my choices 5 times and his opinions are superb.

Florence is an easy day trip fro the Siena villa. And I would keep Rome till the end, it's still warm in Rome in late October. Start with Venice or Tuscany.