I just got my Caymans in the mail today (in lime green, of course!) and I LOVE them already!!!
I wear a 7.5 in shoes. I went to Dick's and the Hallmark Store and tried on a variety of different Crocs. In Beach Crocs, I was most comfortable in a medium (W 8-9). In Caymans, I opted for a W 8.
That's the difference between Beach and Cayman. Beach Crocs are done in a "ranged" size (4-5, 6-7, 8-9, etc.) where as Cayman Crocs are a single size (5, 6, 7, etc.).