I'll chime in also. In St. Maarten, one is likely to see grouper, coral crab, burrfish, lion fish, slipper lobster, barracuda, the largest lobsters I've seen anywhere, and some pretty big reef sharks. Of course, there are plenty of the other common reef fish. Some of the same life you would see elsewhere. There are also a couple of decent wrecks that are fun to swim through.
A difference, in my opinion, is the topography: one would see much more variety in places like Coz or Grand Cayman (tunnels, canyons, walls, etc...) In Coz, you only have to go as far a the Marine Park to find great dive sites. In GC, one could stay on different ends of the island and dive only that side for great dive sites. On St. Maarten, I believe one has to dive the SE and the SW sides of the island to get some variety. Most dive sites are on the Dutch side and I can't comment on the few sites on the French side. A day trip to dive in Saba can also be arranged.
The island itself is, again in my opinion, a great island to visit. It's pretty easy to drive around, Phillipsburg is nice beach town to walk through, and the food is really good.