I've been going to Belize every year, with one exception, for around 10 years now, maybe more. The first two times, my wife and I went to Turneffe Island Resort, which is a small resort on a 12-acre caye in mid-northern Belize. The last eight or nine times, we go to Placencia, which is further south. As much as we enjoyed staying on a caye that was totally devoted to eat, sleep and diving, we enjoy the access Placencia provides to the jungle and southern Belize, where fewer tourists go so we can get a more authentic Belizean experience.
Obviously, if we didn't really like it we wouldn't go there every year. I'm always on the prowl for retirement property there. My wife, a creature of the malls, is a little reluctant but I'm sure will be eventually persuaded. I just booked a trip in June. It's not Nirvana but any stretch but it feels like home to me.
I'm not sure where the mixed reviews came from, but recently a couple of folks were posting here not only that the diving is better elsewhere, which could be, but that the diving in Belize is bad, which has not been my experience at all. Not sure what that was all about. True, I saw bigger turtles in Hawaii, more fish life in the Keys and dove some thrilling cenotes dives in Mexico, but I love the diving in Belize and wouldn't go there year after year if I didn't.
You will definitely enjoy getting your Advanced Card wherever you go. Keep in mind, it's a "sampler" class that allows you to explore other types of diving. The main advantage is that you get to broaden your horizons under the direct supervision of an instructor and that's always a good thing imho. But the name "Advanced" rubs a lot of people the wrong way as it implies that you're way above the skill set you got from your OW class, which is not necessarily the case. I imagine those people would have less of an issue if it were called "Open Water 2" or something like that.
Back to Belize. Small country but still a big place. Think Massachusetts. A very large majority of tourists and divers go to Ambergris Caye, which is fine by me as it leaves the rest of the country to the rest of us. Lots going on there with the most hotels, bars, restaurants and dive shops. Short boat rides to the reef. There are resorts like TIR above that are out on the cayes (pronounced "keys" as in the Florida Keys) that tend to be fairly high end and are like summer camp for adults. Very short boat rides to the reef as they tend to be sitting almost on top of it. To the south is Hopkins, a Garifuna village meets Canadian retirees kind of place. Hamanasi is highly thought of.
A little further south is Placencia, which is another fishing village meets retirees kind of place with more of an island vibe as it's a small peninsula with Placencia village at the tip end. If you go there, I strongly recommend Splash Dive Center. They are my "go to" dive op and where I got my Master Scuba Certification years ago (not to be confused with Diver Master, which is much more of a challenge). Every year I go I see other divers doing Discovery Scuba or getting the OW or AOW certifications, which is always very cool. Ralph and Patty, the owner/operators are good people and can help with lodging and all that. The further south, the further to the reef. Some people don't like that. Me, I love riding in boats.
Punta Gorda is almost at the southern tip. Reef CI runs trips out of there, but I think they're based in Placencia now. PG doesn't get a lot of tourists, which is too bad because the further south you go the poorer Belize becomes and they could really use the money.