Cancun or Grand Cayman?

Ok, I have earned all these Marriott points of which I would like to use at a hotel where my husband and I can just relax on the beach and eat and drink. I don't need to sightsee. Snorkeling I would definately try. I just want a beautiful beach with beautiful water and sand. I have taken vacations where we would hike walk or drive to different monuments, parks, etc. Now I just want to relax without hiking, walking or driving. I have narrowed it down to Grand Cayman and Cancun which both have Marriotts located on the beach. I have researched both, even through Yahoo Answers. But I am still stuck on deciding. Which would you recommend? Cancun? Grand Cayman? In early December?