I lived and worked in the western Caribbean area for many years, and got down to Belize a number of times, over several years. I've dived Cay Caulker and Ambergris Cay, and did a small live aboard with a couple freinds that ran out of Ambergris, and went to the blue hole, half moon cay, etc.
In my experince, Belize has relatively cloudy water and brown lifeless reefs, and isn't worth the cost and trouble of going for diving, versus other options. The same goes for Roatan. I'm not saying they're utter dogshit, but they're hardly impressive. If you haven't been to Cozumel, that's where I'd recommend, for the region. The diving is tough to beat (in this hemisphere), and you get that whole exotic-foreign-paradise vibe, without the cost and trouble of flying halfway around the world. It's also safer and cheaper than many foreign localles.
Also, I haven't personally dived the Caymans or the ABC's, but I had tons of freinds that had worked in both, from what i understand, they're all better than Belize or Roatan (though they're probably gonna be more expensive overall, than belize or roatan).
JMHO, take it for what you paid for it.