Florence is too busy and full of tourists.......Siena and the drive there is glorious. We made San Gim our base. It is truley mediaval and makes it easy to explore other hill top towns for day trips.
Siena has a charm of its own.....its narrow streets, wonderful doors, piazza, restaraunts, Cathedral Art and interesting shops is def a worthwhile visit.

I love BOTH Siena and Florence and don't think you have nearly enough time, but if you are on a tour there's not much you can do about it. What else does the tour include? If it is just major cities (Vencie, Rome?) then I guess I would do the Siena excursion because I think you would want to experience at least one smaller town in Italy. Siena is not tiny but it is smaller and very different from Florence. However, if the tour does any Tuscan or Umbrian hill towns then I would skip the half day excursion. With only a total of two days I wouldn't waste the transit time (over an hour each way). Which ever you decide, make sure you save some time for just wandering around, in addition to going to major sites like museums and churches.

I've been to both several times & definitely would vote for taking the half-day in Sienna. Just to stand in the Campo & imagine the swirling banners, crowds & horses racing in the Palio is really something! The Duomo is stunning - inside some of the most marvellous carving & painting you'll see anywhere. Make sure you go into the little "Library" on the left as you enter - the ancient books are incredible.

Just a thought - you might make better time if there's a train Florence to Sienna. The traffic can be a bit sticky for the first 15km or so getting out. Are you committed to the group tour? If not, this might be an option - and you could wander around at your leisure. Lots of wonderful trattoria in the side streets too.

I think it really depends on the rest of your tour - if it is all big cities, Siena is a slight contrast; San Gimignano would be a greater contrast as it is smaller. Both are wonderful but, as a lover of Florence, I suggest that 2 days is not nearly enough. Perhaps you should stay in Florence and plan to visit Siena and the countryside on another trip.

Another thing to consider - the ride from Florence to Sienna is one of the most scenic in Europe. If your tour is not driving thru much of Tuscany, the 1/2 day Sienna trip may be worth it just for the scenic drive. Also - we loved Sienna, even more so than Florence.

I'll be a dissenter as well. I absolutely love the Duomo in Siena and think the Campo is a great place to stroll and people watch. But this last time there, I realized that's all there was to see there of real interest to me. I didn't get the feeling others did that it was not as touristy or more of a "city".

However, I've been to Florence twice and only feel like I've just started to get to know the city. You seem to have an agenda already that will fill two full days in Florence. If you are firm in your resolve, I'd stay put in Florence. Like you pointed out, half-day tours can run over and you could end up with really only 1 day in Florence total, which would be a shame with all you want to see.

Save Siena for next time, and be sure to see San Gimignano too!

I absolutely agree with the posters who've suggested a trip to San Gimignano. Although it's very touristy in the summer, it's definitely worth a stop, or preferably an overnight when the daytrippers have left town. I found it magical, and much more interesting ( SORRY!) than Siena.