As you will be spending two nights in both Positano and Sorrento on the tour I would look for other places to stay. I did the tour in 2011 and in Positano spent a relaxing day on the beach; others went to Revello and raved about it. You could also do a day cruise to Capri. Pretty town but as someone else said, its mainly hotels and shops. I had been to Amalfi before on a nice day trip. Both Positano and Amalfi are great bases for visiting along the coast. Are you planning on renting a car? Parking in Positano is difficult; I don't know about Amalfi as I was on a boat.
I enjoyed Sorrento and from there you could revisit Pompeii or go to Herculaneum (well worth the visit and I know a lot of people prefer it to Pompeii) or do Vesuvius.
If you stay in Sorrento check and see what hotel RS is using. We stayed at the Ulisse DeLuxe Hostel which has to be my least favourite RS hotel (from 3 tours in Italy). Suprisingly it doesn't tick many of Ricks hotel criteria (memorable, family run, centrally-located hotels). Having said that I notice recent reviews on Tripadvisor are more positive. It was OK for 2 nights, but if you want a view or a hotel closer to the centre of the town, with more atmosphere there are lots to chose from in Sorrento.