For ingame, it doesn't necessarily matter, as proper grinding can defeat the game easily. However, I'd go with Shadow Punch, as it does have more power, and can those potential Double Teamers / Sand Attackers.
Competitively, however, is a different story. Grimer / Muk are extremely slow, and Shadow Punch has horrible utility. Shadow Sneak alleviates their slow speed and gives you a chance to pick off weaker opponents.
