My vote is Caulker over Ambergris if you are the type who doesn't like to think of himself as that typical tourist (even though you know you are). Caulker is, in a word, laid back. They don't call it Caye Slacker for nothing.
As for dive ops, the one I used is gone or switched owners. My wife, on a trip I had to sit out, had a bad experience with Frenchies. Apparently there are dive ops that have you sign up for a trip the next day and then, if not enough people sign up, don't bother to inform you at your hotel or by posting a sign in the morning that the trip was canceled, so when you show up first thing in the morning you are out of luck (in my wife's case, Frenchies was simply closed--with no explanation) and have to scramble to find another dive operator. In all fairness, we have since heard that Frenchies is not the only one that has been known to pull this trick.