Shannon,
You can't go wrong in any of those three locations. If I were to choose one and only one for new divers, I'd probably select Grand Cayman. While drift diving is actually quite relaxing, if your buoyancy skills are still developing, it can be a bit disconcerting. The dive masters do take care of you, but part of the key to developing skills is to not do more than you're comfortable with too soon. Diving with the ship's operator here while good, isn't the best there is to offer.
Belize is good, but the reason Grand Cayman gets my nod is that the dive operators there are extremely professional, high quality and experienced. I'm sure the Belize operators are as well, but I don't have the first-hand knowledge of them like I do of GC's. You will have two wonderful dives in GC without question. Make sure you let the DMs know you're fairly new divers, and they'll take good care of you.
If you want a recommendation for dive shops in Cozumel who will take care of you as novices, let me know. They'll introduce you to drift diving in a small group and ensure you're comfortable. In Belize, I'd absolutely use the cruise excursion, as the logistics to do otherwise have you spending too much time going back and forth to shore - not fun.
Holler if you have other questions!