Advice on 4 cities

Are you driving or taking public transportation? Pitigliano, for example, is harder to reach via public transportation than, say, Cortona. Volterra is quite reachable via a train/bus connection from Firenze (I've done it twice), though it takes a bit of preliminary research to figure out the schedules.

Stay in Cortona and Volterra from the 2 to choose.

Will you have a rental car, or will you be traveling by train/bus?

Viterbo is fair-sized, with a population of over 65,000. It is comparatively non-touristy. There's a walled historic area. It makes a nice change from places that are awash with foreign visitors (likely including at least Cortona and Volterra; I've never been to Pitigliano or Orbetello). Viterbo is not, and doesn't feel like, a cute little hill town; it has about triple the population of Cortona and seven times the population of Volterra.

The Villa Lante outside Viterbo (about 3 miles, walkable if you're active) has nice gardens, though they may not be at their best after a hot, dry Italian summer.

For orientation purposes, note that Viterbo has two train stations just less than one mile apart.

If you plan to day-trip from one or both of your two bases by train/bus and haven't already done so, visit the Trenitalia website to check on travel times to your target destination(s). Day-tripping from small towns via public transportation is sometimes frustrating--especially from hill towns, where your trip sometimes has to begin with a bus ride to the not-so-nearby train station.

Is Orvieto on your radar? Eminently reachable, on the train line between Firenze and Roma, and a wonderful town!

If you're using only public transit, then Volterra and Orbetello might be the easiest combination. I think the second easiest would be Cortona and Viterbo (which I really liked), although one leg involves multiple changes. If public transit is your plan and either of these combinations appeals to you, I can provide more details.

We will rent a car or take public transportation, whichever is most efficient. We will also go to Orvieto - we have friends there. Just trying to get some firsthand knowledge of the other towns and decide which ones to stay in for a few days.

You could easily do Cortona, Orvieto and Viterbo by train. I did exactly that combination of towns by train in 2010. A car gives you more flexibility for side trips, but there are still good options such as Arezzo by train from Cortona. Using this route you would arrive in Viterbo at the Porta Fiorentina station and depart for Rome from the Porta Romana station.

Edit: Here are the trains for this option. 1. Firenze SMN to Camucia-Cortorna (most are direct - frequent buses from station up to Cortona). 2. Camucia-Cortona to Orvieto (many direct, a few with 1 change). 3. Orvieto to Viterbo Porta Fiorentina (all change at the tiny Attigliano-Bomarzo station). 4. Viterbo Porta Romana to Roma (the stop where you'll get off depends on where you're staying - e.g. get off at Roma Aurelia and switch to the Metro to Repubblica stop for Via Firenze hotels).

Cortona and Volterra.

Pitigliano is not very big and doesn't have many accommodation offerings. However, if you are driving by it is well worth a detour both for its unique setting, an evocative town, and its fascinating Jewish history.