It was a trip to Cozumel that sparked my interest in Belize. I've been to Mexico a number of times, mostly on the Pacific side, but when we were in Cozumel a few years agao I realized I could reach another country just by flying a couple hundred more miles.
I love Mexico, most of it anyway, but it's going to be a while before I return there instead of continuing south to Belize. And I doubt I'll ever visit the Pacific side again.
Even though Belize is a small country it's hard to generalize about the place. But there's lots of great diving, good hotels and adventures by the bucket load.
Btw, my definition of "good diving" is warm water, good viz and lots of aquatic life. I find wrecks not that interesting, except for the aquatic life they attract, but the only ones I've dived were intentionally sunk which I think takes some of the fun out of it.