If you're going to make use of a 40 base power dark move, use Pursuit. Far more usable than Thief as not much wants to stay in on Mega Scizor aside from fires, as well as can pick off weakened pokemon or psychics such as Latios, Latias, and Mega Gardevoir that don't want to take Bullet Punch and can't do much back. Overall however, Knock Off is superior in 90% of situations for its sheer utility use of being able to cripple walls, especially Heatran who lacks recovery outside leftovers, as well as weaken wallbreakers and sweepers that rely on a Life Orb to really deal damage.