Group of 15 - should we share small or bigger cars between us or use chauffeurred cars/taxis for airport transfers Florence to Cavriglia and 2/3 day tours to surrounding hill towns/vineyards?
Not a matter of just budget or convenience - if you were to rent 2 vans, are there two people in your group willing to be the drivers? If yes, then you could consider renting 2 vans and moving around on your own.
If you don't have the drivers, then I'd go for the chauffeured option or going for a van+guide to accompany you on the day trips. It all depends on what you'd like to do.
For day tours, also ask at where you're staying at - they might know someone local around Cavriglia or at least closer than Florence they could recommend. It is likely they would give you better rates just because they don't have to drive out there from Florence to pick you up.