I've done both the zipline and snorkeling. I will say that the snorkeling isn't as good as many areas of the Caribbean but it's better than places like Trunk Bay.
The ziplines are fun but make sure you are in fairly good physical shape. There is a lot of walking up steep hills and uneven surfaces. We had a couple of older people drop out because they couldn't take it. However once you do the ziplines you will love them.
I, personally, would go with the ziplines-mule ride-ATV tour but if you aren't prepared or able to do some heavy walking then I would go with the sailing excursion.