How much of an issue is budget? Turks & Caicos has a rep. for being as (or a bit more) expensive as Grand Cayman, based on what I've seen others post.
Land-Based or live-aboard? Both Turks & Caicos and Belize have a rep. for offering up the best diving via live-aboard (though with Belize some might say Turneffe-based resorts would be fine). I'm assuming you're looking at land-based.
Turks & Caicos is often associated with lots of reef sharks in the absence of feeding or baiting; do you like shark?
In live-aboard discussions, T&C is associated with wall diving that starts deeper than some other places; those of you who've done land-based, would that include land-based T&C diving? If so, your average & max. depths might be deeper on a T&C dive trip vs. Ambergris Caye, or am I mistaken on that?
I've only dove Belize via live-aboard, and haven't been to T&C. If it were me, and I had to do land-based, I'd pick Ambergris Caye, because I'd want to go back to T&C later via live-aboard, so as to see what's said by some to be amongst the best diving in the Caribbean, in the best way to see it, particularly at an otherwise expensive destination.
Richard.