+1 for Grand Cayman. There just seemed to be more fish and larger pelagics on Cayman. In a week we saw turtles, eagle rays, schools of tarpon, lots of tropical fish - check out Aquarium as a 2nd dive, there were hundreds of fish there. And fluorescent cleaner shrimp at cleaning stations. My buddy got his cuticles done.
I've also dove Overheat Reef on Roatan, it was sparsely populated by comparison. And maybe a little over dove - we did it with CTDivers, AKR goes there and there almost always seemed to be a boat in the area when we went by.
I agree with don, the North Wall was better than the West. Much more dramatic, slightly eerie when you just hang out over the abyss on the surface - the water changes color, and you can look down and see big stuff way down the wall. And personally I would never dive with Don Fosters (the cruise operator) - most of their seven boats are 18-24 pax. Most of the single boat operators are 6-8 max.
I dove in Roatan a couple years after Grand Cayman. Yet all I remember about Roatan is a great dive at the Texas site and the Dolphin Dive at AKR. Although we did almost always see a turtle or two. Of course on Cayman, that's just about a given anywhere near the Turtle Farm, they've been releasing them there for years.
The water is much clearer in Grand Cayman also. At Big Tunnel I looked up from about 90' and could see the ScubaPro "S" on the safety stop reg 75' above. When you get out on the wall, you can experience vertigo as your eyes strain to see stuff past 2-300' horizontally. See if you can get out to Trinity Caves or Big Tunnel as a first dive with someone. We also really like Hepps Wall - it's somewhere in that area as well. All are just north of SMB so would be considered "West" wall sites afaik. Aquarium is just about in the middle of SMB.
This is going to sound stupid but Cayman also seemed to be brighter underwater. Probably due to the water clarity.