I would say by default to go with Explosion because stat changes are easily reset. The best Memento will do is cause your opponent to switch out, which could be good or bad depending on the battle. I just feel it isn't worth sacrificing your Muk for something so easily reversible, even if Muk is about to die anyway. Damage will obviously have a more permanent effect on the battle (unless of course you battle against a Pokemon with recovery or a ghost). Muk also has a respectable Attack stat, so it will do a decent amount of damage. That's just my opinion though :)