I'll be interested to read what you think of it, Candiru. When people post about the 'best diving in the Caribbean,' seems like Little Cayman is most often mentioned, with Turks & Caicos maybe next, and the Lighthouse Atoll region out of Belize in similar standing. I haven't dove T&C; I dove the Cayman Aggressor with several dives off L.C. as part of the trip.
I don't have a great recall for comparative experience, but overall, I wasn't left thinking either was head & shoulders above the other on diving. Some of the reef life was more accustomed to mankind in the Caymans (e.g.: Nassua grouper who let you touch them, a hawksbill turtle that swam out to somebody), and Stingray City was neat, but I enjoyed the shark dive and the 'land tour' walking around Half Moon Caye on the Belize trip.
I think you'll have a good time, but let us know how it goes.
Richard.