Just back from four days in Ambergris Caye (Sept 11-15). I used Protech for the local dives, and Amigos del Mar for a Blue Hole-Half Moon Caye-Long Caye trip, and were pleased with both groups. I did not find that the DMs, as a rule, did a great job of pointing out things to see; that said, one of the DMs at Amigos made a special effort to encourage a spotted eagle ray to swim towards us (and he did nothing "wrong" in doing that), and I did appreciate that.
Ability to dive Blue Hole or Turneffe depends on how many people can make the trip. Amigos del Mar requires at least ten people, I believe. Peter at Protech arranged for me to take that trip, since he isn't going there right now. The Amigos del Mar Turneffe trip tentatively set for last Friday did not make due to not enough divers.
If Belizean Shores is north of the new bridge, you'll pay a $5US fee for taking the golfcart over and back -- the dirt road north of the bridge is not great.
I think that you'll be very pleased with the local diving at Ambergris Caye. I found a lot more fish life there than during my last visit to Cozumel (Feb 2006); for example, we must have seen fifty+ nurse sharks on the local dives (not counting the ones in Hol Chan Reserve), and a goodly-number of spotted eagle rays.
The Blue Hole trip also was good, in no small part because the seas were very very calm all day. The Blue Hole is...well, as someone said, it's off the to-do list. Good shark action on the day that we visited. The wall dives at Half Moon Caye and Long Caye also were good. The SI after the second dive was spent on land at Half Moon Caye, where lunch was provided; we also got to visit the red-footed booby sanctuary; as one guy told his girlfriend, "Gee, I've never seen so many boobies". If you want to visit the Booby Gift Shop, do so when you first land, because it apparently closes up after the lunch.