I think Metal Burst is better for it. This is because Bastiodon's Attack is so low nothing else will do that much damage. However, Metal Burst deals a set amount of damage (1.5x what your opponent did), so it doesn't get STAB.
Also, since this is X and Y, Bastiodon can't get Avalanche yet. Once the pokebank is up you can trade it from a Gen IV game (where Avalanche is a TM) to a Gen V game, and then to X and Y. But firstly the pokebank isn't operational yet, and secondly it sounds like a lot of hassle.
So, for an attacking move, I would stick to Metal Burst.