First, welcome to FT.

Second, one thing I would recommend is to search for each of those locations in the Marriott forum and you should be able to find a lot of info specific to each one. There is also the FAQ sticky thread that should also have links to threads to most if not all the locations you specified.

All the Marriott's are fairly nice and usually right on the beach. My suggestion is to focus more on the island you want to explore and experience more than focusing on the resort on the various islands. Each island is different and can offer you a different experience. ^

I have been to St. Kitts and Cayman myself. I've never been to Aruba, but I've been told that St. Kitts is like Aruba was 20+ years ago (before it received a lot of building out.) Cayman has some of the nicest beaches anywhere and has a lot of very nice shopping (and playing with the sting-rays is something not to miss.)

I will say though that the "nicest" Marriott resort I have been to in the Caribbean was the Cayman RC (and it is on the nicest part of 7 mile beach.) But again that depends on what you like/want in a resort.