They are both countries, Curacao is but one island, Belize is many.
The diving in Belize is best done from one of the outlying cays, either from an AI located on one of the many islets- or from one of the few day dive ops that might be found on the few of the larger cays.
Curacao is a small island, part of the "ABC" chain, and is well accustomed to AI dive resort ops, as well as hordes of cruise ship day dive people- as well as casual European vacationers who might want a day dive op experience. Lots of dive ops., boats and dive sites. Lots of development, restaurants and entertainment.
The same level of topside jollies would be impossible to duplicate- even if you based in Belize City. The diving isn't as easy from the city, no- it's really quite difficult. The topside frivolity is limited and twitchy. In Belize- you are best off on one of the quiet cays.
With your profile and your stated dive log, you might like the Curacao experience better. It will give you the exposure and opportunity to see well trained DM's at work doing what they do best- pointing out the cool stuff that you and me (city folk) would likely swim right by. Belize is something best savored by divers with many dives, with perfected buoyancy- and for those who understand that treasures don't necessarily have to be past 45 fsw.
Belize is more tribal and free form, Curacao is a more established process. The Bonaire experience I would compare more favorably to Belize- not saying that Bonaire is better than Curacao, just a closer comparison in overall experience. Not to say that drinking Heinikens with vacationing Dutch nurses in Curacao is all that horrible, it's just in Bonaire they're likely wearing less clothing.
Weather? You have the safest bet of avoiding the pestering rains that linger after the Caribbean Huricane season- in Curacao. No real risk of heavy storms at that point, but Belize is much more susceptable to Huricanes-that-never-formed/now NW tracking thrunderstorms... as well as the "Northers" that push down South out of the US- when the cold snaps hit Texas, Belize is about to get it's but kicked.
Each has it's plusses.
