Coz is the clear winner, IMO. I did my very first dive trip in Belize back in 1988, and it seemed impressive then, but even a lake is impressive for your first dives. I went back in 2002 and stayed/worked as a guide for a bit. I know that it has been a while, but I gotta say, I am not a big fan of diving in Belize, nor am I a fan of the Commonwealth prices for everything (pizza, beer, etc.). The inner cayes are dull and boring in terms of diving. And don't be fooled by the marketing that there are nice beaches on the inner cayes, 'cause there ain't. Mangrove, which is cool in its own right, but can be a let down if you are expecting toes-in-the-sand. There are a few nice dives around the outer cayes, but nothing truly great. For example, Turneffe is only good right on the "Elbow". And the Blue Hole is impressive from the air, but not so much as a dive (the nearby Half Moon is pretty Ok though). I did my DM in Coz and guided there for a bit as well, and while it doesn't have much of the BIG stuff (with the exception of seasonal Eagle Rays), the walls are amazing, and the viz even more so. And Coz is a better place to be for a solo traveler, especially with PdC across the channel. Just my opinions...