Renting the car in Florence may not save you any money, because rental rates are likely just as high, and you would also need to add the cost of the train ticket.
The base (=full price) high speed train fare in Standard Class is €50 per person, although one can buy restricted non reimbursable tickets for half that) and, if you have luggage, probably the taxi from the Florence station to the rental agency (there are no rental agency at the Florence station and it's a 15 min walk to Borgo Ognissanti where rental agencies are).
Also, if you plan to visit the Val D'Orcia, or many of the famous towns south of Siena, from Florence you would need to backtrack south toward Rome (That area of Tuscany is between Florence and Rome). Therefore, renting in Rome is probably the most sensible thing to do, unless you can save a bundle by renting a car in Orvieto (Hertz only) or Chiusi (Europcar, Avis-Budget), which are located between Rome and Florence (really close to the areas you will be visiting) and are connected to Rome by cheap Regional trains.
PS: The drop off location of Venice Piazzale Roma may not be open on Sundays, so you may need to drop the car to the far away Marco Polo Airport (VCE). However there is no difference in dropping the car at the airport or Piazzale Roma. The driving force in the price is if you pick up at Fiumicino airport in Rome. Picking up in a city location in Rome will certainly save you money.