One big question that needs to answered is whether you are flying out of the EU after you reach the airport. The international airport requires 2-3 hours if you are traveling back to the US; I am assuming that you have this time allottment figured into your itinerary.
Both Montepulciano and Orvieto are spectacular hill towns. As much as I love Montepulciano, I would probably recommend Orvieto, as it is closer to Rome and it has the spectacular sights of the cathedral and the Etrruscan caves. Also some nice ceramic shops to visit. I believe that you will be there in the midst of truffle season, so by all means stop for a lunch of pasta with fresh black truffles.