If you're planning just to stay in and around Florence, I would strongly
suggest that you DON'T rent a car. Florence is quite small and it's easy to
get around on foot. Besides, you are not allowed to drive in the historical
center - it's closed to cars. (It's also difficult to drive around Italian
cities because almost all of the roads are one-way, and the streets twist
and wind - they are not like grid-like, like in North American cities.)
Finally, parking in Florence is very expensive.
I'd say take the train - it's very easy and quite inexpensive to travel around Italy that way. And there are direct trains from Venice to Florence.
Laurie in Mexico City (formerly from Rome)