I tried both
Castaway Cay experience and Stingray City myself, it's a completely different experience. I love Stingray City way more.
They'll use feeding tray on CC, which I don't feel as "natural" as they are suppose to be.... I also found the snorkeling time on CC real short. I went to SC two times and it was not crowded, but I was there with local tours (one time on boat, one on jetski!), not the cruise tour. May be you should check the Cayman port schedule and see if there are other cruise ships that day. By standing on the sandbar on SC in the middle of the ocean is pretty amazing itself. My wife changed from scared by the ray to love them, she eventually just grab and hold one up to kiss it all by herself!
My buddy who grown up from Cayman Island suggested Turtle farm if your kids like turtle!