Voted for David ...
Several years ago, before our
DCL Med cruise we spent a few days in Rome and several more days in Florence. And at our La Spezia stop, we went to Pisa on our own.
The drive to Florence is several hours each way

- so I would take that into consideration. (less time if docking in Livorno vs La Spezia)
The Academia where David is, is a smaller museum. The David is SPECTACULAR - wow. But there was something about
Michelangelo's "Prisoners(link)" that just absolutely fascinated me. They are very cool sculptures in an "unfinished" state (on purpose). Powerful.
And Florence is a cool city. You won't have time to visit the Uffizi Gallery, but seeing the outside statues is cool. The Piazza della Signoria has a lot of statues. Then there is the Duomo (Cathedral of Stan Maria del Fiori) and its Baptistery with the doors ("Gates of Paradise"). And the "other" cathedral - Basilica of Stana Croce that contains the tombs of Michelangelo, Galileo, Dante, etc.
(and last summer we saw "David" twice - He's in the Victoria and Albert museum in London, and while walking in Copenhagen, we ran into him again!!! But the original in the Academia is the best one by far)
Pisa has the Field of Miracles, with a tower that's trying to fall over. The Tower, the Duomo/Church, the Cemetery are all worth visiting, and I know that we've all heard about / "studied" / are familiar with the leaning tower (that's why we went). Beautiful architecture. But it's not a day trip. Oh yea, I forgot to mention all of the "street vendors" selling cheap junk from China. And the "permanent" tee shirt / souvenir shops. Kind of ruins the wow factor of the buildings and the respect / reverence such places deserve. I really, really really wished that we went to Cinque Terre instead - or even Siena a little farther south.
yep, LOVED Florence, liked Pisa. Will go back to Florence some day. Only going back to Pisa if I want to climb the tower (we did not do this on our visit).