Two completely different properties.
Mirage has no restaurant or bar. No entertainment. Great pool. Great views from the rooms. Great staff. Very clean. Very quiet. If you crave socializing...this is not the hotel for you. It is small and the guests leave the property for food and drink, so you only see them in the morning and then before they leave for dinner in the evening. It is clothing-optional for sunbathing on the decks. I don't know about clothing-optional for the pool, but you will likely have it to yourself, so no worries!
Xtabi has a great restaurant and bar. A better social scene in the evenings. The pool is pretty nice, but not oceanfront like Mirage. The rooms across the street are budget. Mirage doesn't do budget. The snorkeling is better at Xtabi. The entry to the ocean is better at Xtabi. Xtabi is a bigger property.