It may be helpful to establish a few things that seem doubtful to me. (We took a huge, passenger only ferry with hundreds of airline style seats from Capri back to Sorrento a year ago, in the last week of May. It was so full that we had to sit on the floor in the (very slightly) open-air but completely covered rear deck to be with our friends. But it was not a hydrofoil.)
Questions:
1)Do hydrofoils exist on the Sorrento-Capri run, as well as on the Naples-Capri run, which is a much longer distance?
2)Does the travel time from Sorrento-Capri make the mode of boat power very important?
3)Does the OP wish to subordinate choice of departure and return times to speed of the ride?
4)Is advance purchase a)Really important to the OP b)Necessary at the end of May?
5)Don't regular boats occasionally get cancelled for really rough water?
I would note that a quick online look reveals a lot of predatory efforts to pre-book ferries, with vague terms like "High-Speed Ferry", which is a transparent effort to blur Hydrofoils with other types of boat. We walked to the Sorrento marina and found several different companies offering ferries to Capri. Of course, one company will not volunteer that you could leave forty minutes sooner by going around the corner to the window for their competitor. I also felt that we were given pressure to buy our return trip for a specific time, when we bought our outbound ticket.
Now, during a busy time (which is every time the sun shines, in Capri .. ) there is some value to knowing that you have a seat home in the afternoon. Who wants to risk having to wait an hour, or pay a (possible) premium (like a last minute airline seat) to get home for dinner? Despite having a ticket, it was annoying to find that our return ferry (same company, both tickets bought at the same time at the same window in Sorrento) left from a slip 1/4 mile from where our arriving ferry had dropped us off. We should have looked for some sort of display showing what slip the boat would be at, instead of assuming that the same company uses the same slip all the time. I think one reason for this was that our ferry home carried at least eight times as many people as our ferry TO Capri.
BTW, we skipped the Blue Grotto, and were glad we did. There was a more than we could see on a daytrip visit, without spending hours on the Blue Grotto. We also thought taking a cab to get our travel started was a good value, after we saw how crowded the microscopic municipal busses were, and how long the lines and trip intervals were.