We've just got back from there. Combined it with a couple of days in Pisa.

Preferred Florence. A very vibrant and interesting city. Highpoint was visiting San Croce church and if you go then reserve the best part of the day for it.

We went from Bristol to Pisa. Got train from Pisa, €5.20 each, and stayed four nights in Florence at a very poor, in our view, hotel.

Back by train to Pisa and had two days there (nice hotel). Not so much to do except around the tower but glad we went. Very impressive.

Stay tuned and I'll get some pics up soon and hotel reviews.

Edit:
Soggiorno Pitti Hotel (Florence) HERE

Bologna Hotel (Pisa) HERE

Flew to Pisa and then train to Florence. Similar experience, beautiful cities but our hotel in Florence (30km from florence it turned out) was supposed to be 4 star but it was crap. Seemed only to want to cater for coach parties with specific itineries and menus. If I went again I would stay in Pisa and catch the the train to Florence.
Once we worked out that the 23 bus from the station took you to all the tourist spots it was fine. Highlight I thought was the Cathedral Dome, being an engineer I marvelled at the design which mathemeticians have only just worked out how it was done.