If you want to do land-based diving then I agree that the Caymans are a good choice.
There is awesome diving in the Turks and Caicos but I think the best options are reached via a liveaboard dive boat out of Provo; although Grace Bay Beach is outstanding and the water is beautiful.
If you want topside attractions (restaurants, shopping, entertainments), very good diving, and a fabulous beach; then Grand Cayman is your best option.
If you are more interested in a small, quiet island, and a diver-dedicated resort, then I suggest the sister islands of Little Cayman and Cayman Brac. They both have excellent diving and Little Cayman has some of the best diving in the Caribbean IMO.