OUr last trip we did the "wild thing" and that took us to St. Johns, but I understand they usually go to a different snorkel spot, so we were pretty lucky. Haven't done either of those tours, but as long as you get to St. Johns, its a lovely beach and the whole island is a paradise, so just make sure you make it there. Once you do decide on which one you want, you can pre-book the excersions through the Princess web-site or mail, fax you requests in. We were able to get what we wanted by doing it early.
What Princess ship are you cruising on? Have a great cruise:-)