I've stayed at the Cayman RC and my wife has stayed at the Westin before. They are right next to each other on the best part of 7 mile beach. The Marriott is up the beach a bit. The beach there is somewhat smaller and a bit more rocky.

We did walk up to the Marriott while there and look around. I rate the hotels as follows:

First: RC, by far the best of the three with a couple of pools, great beach, great service, great food, etc.

Second: Westin. My wife really liked the Westin and given it is next to the RC, it has a great beach location as well.

Third, Marriott. Not quite as nice as the Westin, not as good a beach and a smaller pool. I wouldn't say it is bad, but compared to the other two, it's not quite up to their level.

I would also echo aaupgrade's thoughts on using Marriott points to stay at the RC. The RC there has been very good about having Pointsaver award availability (that is what we stay with) which makes for a great use of your points. We did a 5 night stay for 150K points. If you have the points, its definitely worth it. ^