Can't tell you specifics about either but excursions do sell out and that may be the case with the one you preferred. You may find that at some point it will become available again, they sometimes have the ability to add another bus if demand is still high. The problem with waiting to see is the other choice may also fill up while you wait. Call the cruise line and see if there is a waitlist available for choice 1. Also find out about if you book the second can you cancel without penalty if the first does in fact open. If so book the second and add your self to the first.