My experience / belief is that a 3D decoy is best when set in place.....but if I was stalking/decoying, I like the idea of the easy fold up, realistic image etc....I can also always show a broadside to the animal I'm hunting, I've used some 2D decoys before and they get attention but once the deer came to the side and saw it paper thin they spooked, or were weary.

So for stalking, it would be my choice.....if leaving in place, I'd pick a 3D decoy instead.