I would go for Cancun.

Flyerwife is correct about the Nassau Marriott. We stayed there last May for 4 nights. Conditions were bad enough that I wrote a complaint letter (after complaining to the manager). Problems with service in the restaurant, quality of food, cleanliness of our room upon check-in and after daily cleaning, trash on the back stairs down to the pool/beach area, etc. I'm guessing that the issues were not isolated, because I received four certificates for free nights w/ breakfast at any Marriotts. Our stay reminded us very much of the Bermuda Marriott, two months before it was closed – permanently.

We stay at the San Juan Marriott for a night or two each year and generally like it. But the beach isn't great - too much shade in the morning. And the location isn't great either. A number of nearby restaurants, but not much else of note. Still, with a rental car for a few days and use of taxis and public transportation the rest of the time, there are a lot of nearby things to see and do.

I can’t say much about St. Thomas. There is a lot of ship/boat traffic in front of the hotel. Great setting on the other side of the hill from the cruise ship dock. A short cab ride to downtown. We have been to the hotel, but haven’t stayed there.

We have had the best service at the Cancun Casa Magna (three separate stays) of all the Marriotts we have stayed at. We have not stayed at the adjacent JW. Although the JW is currently undergoing renovation, that should be finished by the time of your trip. The only negative is that these two hotels are not within walking distance of the restaurant and entertainment centers. But taxis are plentiful and inexpensive. You really don't need a car.