I did snorkeling in Cayo Guillermo (next to Cayo Coco) last year and I had a great time. I cannot compare, it was my first time. We went to 2 different shallow reef, both were about 1-3 meter deep.
I have seen tangs, butterfly (only few), angels, triggers, snappers, barracudas...I couldn't be happier. But like I said I cannot compare...