Ravello is generally accessible only from the town of Amalfi, so you change public busses there. Ravello is high on a hill.
Perhaps it will be less crowded in September than at the height of the season, but Positano is always crowded. The SITA bus line in Sorrento can be more people than will fit on the next bus. In high season, Rick Steves (travel writer) reports that the SITA bus back to Sorrento, which originates in Amalfi, sometimes fails to stop in Positano on the way back, because it is already full!
You didn't mention the most expensive option, but some people hire a car and driver (because it is so hard to park, and the single two-lane road is so crowded.) That's what we did, 245 Euros for 8 hours.