Florence is a matter of choice. It's about museums, and seeing famous works of art. Otherwise, the city is nothing special or beautiful other than sunset on Michelangelo hill, or a short walk along the water. It's renaissance art fans glory. But if you want to see archeology, the Forum, the Coliseum, the baths, the Rome the way i was 2,400 years ago instead of seeing some paintings, it's a personal choice. I personally would spend a few days in Rome, get to the Uffizi and Accademia early to see David and a few others that you are interested, and spend the rest of the time exploring the back streets of Venice. Squeezing in too much is the worst thing you can do. Art fans, Rome and Florence. Want to know what makes Italy special, a few days in Rome, then a five hour train ride to see the country side en route to Venice. And when you get there, find out where San Marco Square and the Rialto Bridge are, and go in the opposite direction unless it's early or late at night. You will have great memories.