I would pick Grand Cayman Island. I have been to Mexico many times and enjoy it, but there are better places in the world. I have never been to the Caymans and am very interested in their government and their relationship, as a British Dependent Overseas Territory, to Great Britain.
As for diving, I have never dived that part of the Carribean and am interested in learning about the differences in currents, sea life, and weather.
My first choice for a hotel/dive operator would be Sunset House. I want to visit this dive resort because of the good press that it gets in dive pubs and on websites.
Gin Clear Water To Ya!
markm