Florence : 3-4 hours

There is just too much to see in Florence and I doubt that anything super worthwhile is open after 5PM. I would go directly to Venice and recover from your flight. And by all means, put Florence on your bucket list. I've visited three times and hope to return.

Leaving and retrieving your luggage in Florence will take time, it's a very busy station. I would leave it for another time, sounds like you are already planning to return.

If you don't have at the very minimum two nights to stay in Florence, don't bother. Just go to Venice.

However if you have lots of nights scheduled for Venice, you can probably borrow a couple for Florence. For example if you have six nights available you could easily split half and half between the two. Florence has more to see than Venice. Also Florence can be used as a base to visit other towns in Tuscany.

Both Florence and Venice are wonderful and beautiful cities to visit. Seems to me that a good answer to your question depends on various things: what month you are traveling, thus what time it gets dark; will you arrive in Pisa already exhausted from a long flight; how many nights do you have scheduled for Venice, and can you shift one or better two to Florence; how much baggage will you have; can you reserve a room, ideally close to the station, etc. I myself could, and have, spent hours just looking at and photographing the Florence Duomo, Basilica and Belltower complex, from various angles, and during daylight, sunset hours, and evening, when it is illuminated. Central Florence is compact and the RR station is close by, and you could enjoy strolling around for a few hours, even without entering any of the beautiful buildings, many shops, and fantastic museums.

Go onto Venice

add a second night to Florence. are you taking time in Pisa to see the leaning tower?
in the evening in Florence you can have a great dinner, walk up and over a bridge, and also photograph the statues out side of the Uffizi.......

3-4 hours are very few but if you want to do a short tour in Florence just a few minutes from the railway station there is the famous Piazza del Duomo, here there is the Cathedral of Florence an unique example of architecture with the Cupola of Brunelleschi and the Baptistery. Those are the most beautiful and important monuments in Florence and it's very close to the Central Station from where you can easily take the train to Venice.

http://www.florencetourstuscany.com/cathedral-of-florence