Sorry, but I have a weakness for semantic arguments. While both terms are commonly used, I beleive the original term was "duck" tape. It was invented to waterproof ammo boxes for the military, hence "duck" as in waterproof, like the animal, long before being used on ducts.

I don't mind a little semantic discussion myself. If you go to Wikipedia , you will see that they agree with your first use of the tape for ammo boxes, but they say that the name came later, when it was used on ducts, not ducks...