You are referring to Labadee, not Coco Cay. Labadee is a private area on Haiti, Coco Cay is a private island in the Bahamas; both are utilized by
RCCL.
I've been to both Castaway and Coco and I think they are both pretty. I preferred Coco Cay but it really depends on what you want. Food was about the same on both (yuck!), beaches were very pretty and clean. I did like that there were trams on Castaway to get you around from place to place. Our
DCL cruise with kids couldn't dock at Castaway so we never visited the family beach, our Fantasy cruise was sans kids and we went to the adult beach. I thought the loungers at the adult beach were poorly maintained and drink service was spotty. Coco Cay is a tender port so that's different, if you don't like tenders, then Castaway is the better option.
I've been to three private islands (so far) and they are all lovely beach days. So far, Half Moon Cay has been my favorite but I wouldn't turn down a day at any of them!