We will be in Italy for 10 days in the spring, primarily in Rome to visit our daughter who is spending the year abroad. We would like to spend a night or two in a smaller, nearby town-- we were thinking either Spello or Orvieto. We will not have a car and have no problem walking a lot.
Spello is smaller and I had more of a sense of going back in time there; at night you can practically feel the ghosts of the past around you as you walk the narrow streets. Great views out over the valley; I remember a lunch in the back courtyard overlooking those splendid fields and hills. However when we were there we had a small car, so I'm not sure how accessible it is via public transport. It is quite close to Assisi; if you were staying in Spello I'd suggest looking into the buses for a day trip to Assisi.
Orvieto is larger and easy accessible via the train. It has a large church, a tower you can climb, and a famous old well you can descend, lots of local goods (pottery, etc) for sale. A pleasant place to visit. If you're going to Orvieto, get a copy of "The Lady in the Palazzo" by Marlena de Blasi, the story of an American woman married to an Italian from Venice and their move to Orvieto.
You can't really go wrong with either choice.
Orvieto without a doubt! Spello is charming, but within an hour we were bored. We spent an entire day in Orvieto and still never saw all that we wanted to see! The cathedral (Duomo) is one of the most impressive in Italy. Inside is Luca Signorelli's fresco cycle which makes the whole trip worthwhile!! This fresco is said to have inspired Michelangelo's Sistine Chapel fresco. Orvieto Classico is one of the best dry white wines in Italy! If you have time, do the tour of underground Orvieto...Etruscan era. Then there are the incredible Italian ceramic shops. We all decided these were the best we had shopped all through Italy!
Orvietto has a lot to offer. The Cathedral was built to honor the Corpus Christi miracle and the tunnels under the town date back to pre-Roman, Etruscan, times. The town is built on sheer cliffs rather than a hill. It was a pleasant surprise between Florence and Rome.
I haven't been to Spello so I'll let others describe it.