This May, I did a week trip at CocoView in Roatan followed by a week on the Peter Hughes liveaboard in Belize.
Now, I really loved the liveaboard in general, but I also have to say that I thought the diving in Belize was very good, easy, and moderate profiles (except the Blue Hole which I thought was a waste of time- I went deep, had relatively poor viz, saw some interesting rock formations and found a nudibranch- pretty much like a shore dive in Seattle).
It was similar to Roatan, with much more fish, turtles, rays, etc. The walls at CocoView are bigger/longer and have more coral. We did a day trip to the other side of Roatan, and the reefs there were more similar to Belize (and, in my opinion, better than the ones at Cocoview- if I go back to Roatan, I think I'll stay on that side of the island).
I think "Aquarium" may be my favorite dive so far. I hope your lucky enough to have as great a trip as I did.