Florence can be so crowded and humid (I've nevertheless been to a Venice but I'd expect it to be very similar) so it all depends on whether you want a break that is about Seeing All The Things, or one which is a bit more leisurely. If you like just wandering, sitting and people watching, I'd recommend Lucca or Siena - both smaller, both beautiful, and just that bit more laid back - with the understanding that while both have beautiful and interesting bits of art and culture to see, they don't have the Uffizi or equivalent. However for me, the people watching and just being in Italy outweighs that.
However if you want to have loads of museums, galleries etc, then Florence is the best bet (or Venice I'm sure, I just haven't been there like I said).
If you do Foorence, I strongly recommend that you see the Boboli gardens, which are beautiful, and climb to the top of the cathedral - it's a bit tight getting up there by the view is work it every time 
Nice dilemma to have, enjoy wherever you end up!