Both towns book up six to twelve months in advance. Don't assume you will be able to get your first choice. Make sure of your onward plans (timing) if they are from Positano, which is much harder to travel from than Sorrento.
Because I couldn't get my first six choices, we stayed at the Ambasciatori in Sorrento, which I think Rick describes as a "worthwhile splurge." It's a little too far to walk with luggage from Circumvesuviana (taxi 15 Euro seemed a little dear, however), but easy enough to walk to the center of town for dinner. Very luxurious, fierce air conditioning, and (extra pricing) fabulous, magnificent views. Swimming platform in the gulf, accessed by elevator in this cliffside hotel.
As others noted, be clear about luggage services and step count in Positano. We didn't sleep there, but did visit a famous and very expensive hotel, La Sireneuse. We didn't see the rooms, but their balcony bar and pool deck was underwhelming. The pool was about 8' x 16'.