Blue Hole is different. Not much life down there except the sharks which circle on the edge of visibility in the middle. You have to go down to 30 m to see them and the stalactites so it's not a long dive. Careful going down though, I hit 40 m before I realised the beeping was my computer trying to tell me something.
The highlight of that trip however was The Elbow at the bottom of Turneffe Atoll. Amazing viz, saw a sailfish above us, got close up with a turtle, watched dolphins chasing each other, and good coral with all the small stuff that goes with it.
Gladden Split is all about whale sharks. You wander about in empty blue water for ages hoping to see one, and when you finally do it is as fleeting as it is amazing. The snorkellers up top got a better view I think but they also got seasick from the swell as it is outside the reef. The smart ones brought pills and took them on the way out.