Been to Costa Rica and just got back from Belize a little bit ago. Broad brush strokes overview: If you don't dive I wouldn't find Belize that interesting of a place (did not go interior Belize just stayed on Ambergis and Caye), its really a relaxed vibe and not a lot of night life that I found (I was there just at the beginning of shoulder season/end of high season). Beaches are not great with the seaweed problem, but the bar the end of the dock vibe is cool imo. But if you do dive its some of the best diving Ive done compared to (Cuba, Seychelles, Thailand, South Africa) Costa Ricas got a lot of shit to do with varying terrains places to party and places to not. So the real question is do you dive or not? If not go to Costa Rica.