This is a hard one. I have been to both Bonaire and Belize within the last 12 months. They are 100% different from each other. It's like comparing apples and oranges.
In Belize we experienced several sharks, and rays, turtles, and large grouper. I personally like 'big fish'. As for the restaurants and shopping, I would say it is somewhat more laid back with less choices then Bonaire, yet the choices you do have are very good. Assuming you are coming from the US, Belize will price out less, then Bonaire.
Bonaire has the shore diving, and that's the biggest plus there. My group didn't see any 'big fish' on this trip, but there is still tons of coral and small macros that will keep your interest. And the food was very good. The local people in both places exceeded my expectations.
You can't go wrong either way.
:sunny: