Having to make these choices are always difficult, but, I would have to say Florence.
I did enjoy going to Pisa & seeing the Tower, but, I think Florence has SO much more to offer. To mention afew great places to see:
The Accademia Museum where David "lives", the Doumo, Churches like Santa Maria delle Fiore and Santa Croce, Piazza Signoria, Shopping along the Ponte Vecchio................
Assuming that you are docked in Livorno, a trip to crowded Florence and back can be a bit of a push. Here's another choice - using the train, go to Pisa (where a casual stroll from the train station to the Tower is quite interesting)and then return to the Pisa station and go to Lucca to walk the walls of this delightful Etruscan town. Have a gelato across from the station and head back to Livorno.
Definitely Florence. Pisa is a very comercialzed area with lots of vendors with cheesy souveniers and it takes all of 30 minutes to see the tower and pay to climb it...I felt like I had to see it also, since, I had heard about it, but if I could only choice one of the two, it would definitely be Florence - it is a beautiful city, lots of history, great museums and churches. Enjoy!