I can tell you about the 4 Sandals I stayed at.
Whitehouse: you already know that one, that's where I got hooked on diving.
Dunns River: standard Jamaican diving, there's a wreck in the area, nice to see,
Royal Carribean/Montego Bay: two resorts that are covering the same area, you will get better diving there, nurse sharks, eagle rays, big lobsters, the possibility of meeting some local dolphins, two air planes, a wall dive, an area where you swim between huge coral columns. Royal has a smaller boat as of march 07. Very satisfying diving, oh there's a tug boat wreck and a swim through called the Widow maker, say hello to Raymond at Royal Carribean.
Negril: very good diving, different landscapes, dolphins, no big fishes but a lot of colourful ones.
It pays to get friendly with the dive staff, if the group of divers is fairly competent the will take you to some nice spots they don't take every one to.
If you go to Negril, say hello to Tina and Josh for me.