You need the AC3 ticket for Sorrento - Praiano, which costs 2.4 euro:
https://www.ravello.com/sita-bus-schedule/#ticket-types
Runs on time as much as the traffic allows it to, which in late May is lottery.
As the other commenters said, the buses will be packed-full in that period. Trying to reach Amalfi from Praiano on a SITA bus will be a though job. Only in the early morning this is still a valid option. With a car you have your seat on the car guaranteed, as opposed to the bus, but what do you do with the car once at the destination? Parking anywhere on the Amalfi Coast is a nightmare.
Praiano - Positano is linked via an additional bus service: Mobility Amalfi Coast. They publish their timetables on Facebook. Here's the latest one valid until 15th of June: https://forum.amalfi.com/t/mobility-amalfi-coast-bus-positano-nocelle-praiano/26/4. This one is still over-crowded, but you have better chances with it than with the SITA buses for Praiano - Positano (and return) connections.
The ferries are the most recommended mode of transport on the Amalfi Coast. Too bad Praiano isn't serviced, because it's a gorgeous town to spend your holiday in.