Do you plan to rent cars for use during your stay? That will make a big difference in where it is most practical to stay. How long do you plan to be there? At what time of year?
ViaMichelin estimates the driving time from Florence to Venice at 3-1/4 hours, but that doesn't include any stops, getting lost, looking for parking, or getting yourself from the parking place to the part of the city you want to see. Folks who drive in Europe (I do not) suggest adding at least 20% to ViaMichelin's time estimates (taking this one up to nearly 4 hours). Depending on where you stay in Tuscany, your drive could take a bit less time or much, much more.
Because of the availability of fast Frecciargento trains between Florence and Venice, the trip takes only 2-1/4 hours by train. Staying anywhere else in Tuscany will substantially increase the travel time by train, because you'll have to start the trip by either driving to a station served by the fast trains (which would probably mean Florence) and finding parking or taking a much slower train.
The top tourist spots in Venice (Rialto Bridge and market, Piazza San Marco) are overrun during day-tripping hours, so a one-day visit from Florence is probably going to be a disappointment.