All the towns you cited, plus the two I added, pretty much exhaust the list of places worth to visit in all of Umbria. You could add Todi also. Assisi is probably the place I'd stay. Actually it's the only place in Umbria where I've stayed overnight (more than once). It's really central of all, and I've used it as a base to see everything else.
Deruta is not really a big deal for me, but my wife drags me there every few years to buy some ceramic pottery. Gubbio is also famous for ceramic pottery. If you care about pottery, visit both, if you don't care, then Gubbio is more worth of a visit than Deruta.
If you choose to stay in Orvieto, which is very nice but not central to the others and away from the rest of Umbria, you could consider visiting also Civita di Bagnoregio, one of Rick Steves' favorites (but not really a big deal to me). Civita is not technically in Umbria, it's in the Lazio region, but not far from Orvieto.