Driving from Orvieto to Sorrento or taking the train.

The drive is easy, almost all freeway except for the last 3 or 4 miles from Castellammare freeway exit to Sorrento, which is a two lane highway.
I wouldn't bother with the trains, especially if u have luggage. Two people will also spend just as much in train fare.

I agree with Roberto, driving is the best way to go. Last time my wife and I were in Italy, we took our GPS with Italy maps installed, and we had no problems at all -- Venice to Cinque Terre, then to Orvieto, then to Amalfi Coast, then to Sorrento, and finally to drop off the car at the Naples airport and fly home. I actually forgot to take a map with me on that trip and I never missed having one. I find it can be very helpful to do some research before you go about where the parking garages are and how to get to them (Google maps), and use the Google maps street view to 'see' what the streets approaching and at your destinations look like. You can even enter/save some of the points that you want to get to into the GPS in advance. To do that, you may have to switch to the overhead map view on the GPS,then zoom out a lot and slide over to Italy and then zoom in again, then find your desired destination point on the map and save it as a favorite.
Anyway, it will be much less hassle to drive, than to take the train and do all the transfers. I agree with Roberto, the train idea would probably cost more as well.

Our thanks to Roberto and Roger for the very helpful advise. We'll keep the car and drive.