John & Tessa,
IMO, the biggest issue with leaving your boat in any of the marinas
in the Azores
is getting a berth on a floating dock
rather than rafted up on a seawall or hammerhead. The marinas
in Horta and Ponta Delgada are usually very full during the summer months and the only option may be to raft up. Not how I would want to leave my boat.
I believe the best advice is to try Terceira. I think you will have a better chance of getting a berth in Angra do Heroismo or Pria. I don't know about flights from Terceira, but the worst is you will have to fly to Lisboa and change plans there. I seem to remember that the direct flights from the Azores to North America were always booked several months in advance.