I'll vote for the train as well, based on our trip in Sept/Oct this year, we got a eurail pass and loved it. But, as you are going from Venice to Florence, try and reserve your ticket ahead of time from St Lucia station (right in Venice, by the Ferrovia vaporetto station), and try to ensure you don't have to change at Mestre, as carrying your luggage between the platforms is a real pain.

As for the reliability of the trains in Italy, it's perhaps not as good as one of the previous posters said. On our way from Rome to Venice, we had one train switch on our reservation, I think it was in Verona. For some unknown reason our train arrived almost 15 minutes late despite leaving on time without any delays, and we saw our connecting train roll away, on its way to St. Lucia station! We just stayed on our train as it was going to Mestre, we thought it would be easy to get any train across the water to St. Lucia station, and the platform changed at least three times for trains that were to be going to St. Lucia. All announcements there are in Italian-which isn't all the surprising-but the english skills of the workers at the station (while far better than my Italian skills) were pretty much useless. Eventually we ran into a couple who happened to be going to the same hotel as us on the platform....sometimes you get lucky! Enjoy the trip....