Where to stay
Buongiorno,
Where you stay might also depend on what you might want to be visiting - for example if you are looking to visit more of the southern part of Tuscany (like Val d'Orcia, Creti Senese, Arezzo & Cortona and even Chianti) then Siena is a great area to look for accommodations because it will keep you in the general area and make day trips less km to drive. This area is more countryside, small hamlets (great wine) and scenic vistas though you will find lots of history, culture and delicious food as well.
However if you are more into visiting the museums, cities and "hot spots" of Tuscany then closer to San Gimignano and Montespertoli will probably be more comfortable for your driving. You might want to look for a place more in the countryside because I find that the summer evenings under the Tuscan stars are gorgeous!
Buon Viaggio
Donna Denise