On a river dry fly fishing is harder to do propoerly than wets.
If you take a novice onto water you can put a rod in their hand and let them swing downstream wets, they will by accident if nothing else catch small fish. I dont feel the same with drys, its all about pin point acuracy , matching the hatch (sometimes) , good casting and avoiding drag.

On Lochs, Loughs or Ressies i am not as sure.Some one with more experience can comment.

My answer is therefore Drys on running water.