If you really cannot decide you should check if you have time to do both of them. In PV we did a morning canopy tour and still had time for horseback riding, and in Cabo we did the early dolphin swim and still had time for snorkeling. It all depends on what your ship offers (or what times you can schedule yourself) and how long you are in port. But you should have enough energy to do both if you have the time (and money).
Carnival Spirit - Exotic Mexican Riviera - January 9, 2008
Carnival Pride - Mexican Riviera - August 5, 2007