Having done 2 Nekton trips and no other liveaboards I also agree on the choice of Nekton for a Belize liveaboard.
When we did the Belize Nekton trip we would often see the Peter Hughes and Aggressor boats, so no matter which liveaboard you pick you will end up diving the same great areas.
I do want to mention that one of my fellow travelers on the Belize trip had dove on another liveaboard (Aggressor?) and commented about how sick he'd been in rough seas, whereas this was never a problem on the Nekton.
While I'm sure from other reader comments in many other SB posts that most of the other liveaboards offer first rate operations, there really are some significant differences between the Nekton operation and every other liveaboard that you'll have to consider:
Nekton: Most stable (non-vomit) boat out there, plentiful good food, never feels crowded, good (nothing fancy) cabins, great and super competent crew, bring-your-own liquor, economical. (Based on personal experience).
Peter Hughes and Aggressor: Less stable (more vomit) monohull design, gourmet food, probably feels a bit more crowded, nicer cabins, also excellent crew, they supply liquor, more expensive. (All this is based on what I've heard).
So just to let you know that I'm not some shill for Nekton, I'm seriously considering doing the Cuan Law liveaboard in the BVIs. but only because the BVIs are not on of Nekton's itineraries. And when I get to Australia one of these days, I'll probably be on a Mike Ball boat. But in this part of the world I'm diving Nekton whenever possible.