I'm in the process of planning a late spring/early summer trip to Umbria and I'm looking for a nice hilltown "home base" from which we can take day trips. Right now, Todi, Spello, Spoletto and Trevi are towns that I'm thinking might be possibilities. Any thoughts among those or other suggestions? A few years ago, we used Montepulciano as a home base to day trip in southern Tuscany and I'd like to find something similar in size and character (relatively small and not super touristy).
Marcus, I can comment on Spello as we used it for a base last month. It is very small and untouristy with some very good and reasonably priced restaurants. We did not have a car and did fine, but a car will surely give you more freedom. Parking appeared to be plentiful, although in many cases you will have to walk a bit from car to lodging. Ask at your hotel/apartment/B&B what the situation is. We had a fabulous apartment with a view of the valley. Assisi is an 8 minute train ride away, 20 or so by bus. We daytripped to Assisi, Perugia and Spoleto. Really liked Spoleto, but it was a bit larger than we wanted to stay in. Great town though. If you visit Spoleto, be sure to hike across the Ponte della Torri for some magnificent views. Hoped to make it to Gubbio, but decided lacking a car that was a bit out of range. If you decide on Spello, Hosteria DaDa and Il Trombone were our favorites for dinner. Bar Bonci was a great spot for breakfast, lunch and aperitivo.
Your list of towns above are just fine. Assisi is great as is Perugia but they are a bit bigger. Laurel's comment is also great and ties in to your thoughts.
Of the cities you mention as possible bases, I'd say Trevi is a bit remote in terms of travel time up and down the mountain. Spello or Todi would be my favorites. Last year I based in Assisi and it was great, but I don't mind being surrounded by tourists.