Hmm, well Meteor Mash benefits from the Hone Claws more, and it would do devistating damage with 150 BP after STAB, and a couple of Hone Claws. Bullet Punch is nice because it gets a little damage on the opponent before etagross faints, but it wouldn't be used much more than that. For this set, I think Meteor Mash is the better option, though either move would work well for metagross.