Stan struggles against fast pace, djokovic not at all. I think that says it all. Although djokovic has a more complicated swing the answer is Djokovic. You can see in the slow motion how Novaks arm is relaxed and he is ready to accelerate towards the ball right after beginning the swing, while with Stan, his wrist struggles to turn the racquet around and swing at the ball, after the takeback (the motion from 5 to 7 seconds into the Stans video - Novak doesn't have it that tense and strained). Being even more accurate, I think Novak accelerates throughout the whole motion, while Stan fits the most part of into a wrist twist, which doesn't work against pace and serves! Stan slices many returns back on the forehand!