I've been to Belize. Haven't been to Bonaire, but I have been to Curacao, similar diving. These were the two best destinations I have been to for the diving. If that is the choice you have to make, it will be a tough one.
Where are you thinking of going in Belize? My diving there was aboard the Aggressor, all on the atolls. If you are planning on diving Placencia or Amberigis, the diving is different. Spending the better part of a week diving out on Lighthouse Reef was AMAZING!!! If you are shore based, you probably won't be diving 5 times a day as we were on the liveaboard.
Bonaire is pretty much the home of unilmited shore diving. I can't wait until I get my chance to head down there....
Wherever you wind up going, don't forget to post a trip report for all of us to see! Happy diving!