Jenn, any of the towns you listed would be a good choice. However, with only two days I don't think you'll be able to see more than one or two of them. Which towns to choose will probably be determined by transportation (since you won't have a car). One point though, I was a bit surprised that Siena wasn't on your list?
My preferences would be to consider two towns that are reasonably close, for example Siena and Orvieto (which is actually in Umbria). The trip via train is fairly easy between those two hill towns, and if Rome is your next stop that's an easy trip from Orvieto.
A lot will depend on where you're going after the hill town visit. If I were planning the trip, I'd probably spend the two days in Orvieto, with a possible side trip to Civita di Bagnoregio (one of Rick's favourites!).
Happy travels!