Depends if you're using him for competitive play or for in-game.
Stone Edge will almost secure 1HKO's on flying types, one of Mienshao's weaknesses, despite it's 80% accuracy, although Rock Slide still does the job for in-game battles.
All-in-all I suggest Stone Edge as it suits Mienshao's High Risk/Reward play-style.
Also, may I suggest Poison Jab for the newer Fairy-type weakness instead of Knock-Off? :D
Have fun :D