Many thanks, good people.^ That's already given me quite a bit to think about.

We used the ferry last time and it was fine. (Though for the return from Ao Nang to Phuket we actually hired a car and driver and for a reasonable price enjoyed the very scenic drive through Phang Nga.) But due to airline schedules it could mean having to stay one night in Phuket or Krabi on both the inbound and outbound journeys. I'd like to avoid the time wastage that breaking up a jouney like that entails, if at all possible.