We loved Montepulciano but yes it is very steep. In fact, our landlord came to meet us where we'd parked our car and he had to stop and sit a couple of times before we reached our apartment (he was not entirely well). I haven't been to Monticcello, but we did drive to Pienza several times and liked it very much. It's very flat and you'll have no problem walking but it is also quite small.