I was there back in 2009, so it's possible things have changed since then, but you couldn't pick two more different towns in Tuscany than Monteriggioni and San Gimignano.
Monteriggioni is tiny and charming. Because it's so small, its charms are limited, but so are the crowds. When I was there (an early weekday morning in May, just as the town was coming to life), I was one of maybe a dozen tourists. I pretty much had the place to myself. I don't know if it gets more crowded as the day goes on. The views from Monteriggioni are quite beautiful if you climb up on the walls.
San Gimignano is big and wonderful, with grand towers (at least grand by medieval standards). It attracts big crowds and tour buses. It is possibly the most impressive of the hill towns in Tuscany to see from a distance, but it was one of my least favorite to visit, partly because of the crowds, and also because it felt to me as if it had sold its soul to the tourist dollar to a greater extent than any other town I visited. (More tacky souvenir shops, at least it seemed to me, than anything authentic.) I wished I could have stayed into the evening after all the tour buses left to find out if I could discover a more authentic place.
I hope this helps give some perspective.