Hi highly,
We were in Positano in March and I had checked out getting a bus from the airport or a cab and we opted for a cab. To be honest, its a long journey and I felt it was much better to get a cab. I emailed our hotel in advance and asked them if they could arrange someone with a fixed amount and they did it no problem. With luggage, I really think getting the bus is messy and really not worth the hassle.
I wanted to go to Positano for years. Its amazing, a really lovely place. I preferred it more than I did Sorrento although found it very expensive to do any kind of shopping. But the views are spectacular.
Btw, as far as I remember, the cab was about €100 but we were driving for a pretty long time, I definitely felt I wasn't ripped off and the comfort of having someone waiting for you is great. He even stopped on the way back to the airport a few times to allow us to take photos, he knew all the great spots.