should definitely stay in firenze instead of pisa, there is so much more to see in firenze. moreover, firenze is between siena and pisa, so it is a good place to stay if you need to make day trips to these two towns, siena and San G is much farther from Pisa than in Firenze. I think when it is mentioned that one should take a bus to siena, it really means from Firenze.
BTW, don't just shop in Milan, the duomo is beautiful and there is also the last supper painting....
http://hk.myblog.yahoo.com/gogoma-blog/index?l=f&id=6
