PWMTrav's last post is really on target about Florence vs Venice by their attributes. if you're into art, Florence - if you (or the kids) are into Science, the Galileo museum is pretty cool and a stone's throw from the Uffizi.

Venice - a city where getting lost is an art form! Fascinating old buildings standing in defiance of the sea. Murano is worth the vaporetto ride if you like glass products.

We went beginning of June for 3 weeks. The crowds weren't bad the first two weeks, picked up the third when we were in Venice. Busy but manageable from my perspective. I wouldn't want to see what July is like.