The answer really would depend on how you want to spend your trip. If you are simply planning to go from Venice to Florence, and do not care about spending time to see whats between those two cities, than you should take the train. The train station is very close to the port so this is quite convenient. However, if you want to see more of Italy than just cities, a rental car is a very good option. Keep in mind that a car will actually become a liability inside of Florence since traffic and parking are problamatic. But, there is a lot to see and do in Italy outside cities, which is why we almost always rent cars during our visits.