Sturdy is almost useless with Stealth Rock everywhere. Just go with Rock Head for recoil-free Head Smash.
Heavy Metal is eh. Sure, you're practically guaranteed to have a max-power Heavy Slam (and Low Kick/Grass Knot are already maxed out on regular Aggron, so that's not a drawback), but aside from its accuracy, Head Smash is a much better move than Heavy Slam.
EDIT: Oh, I didn't realize this was for in-game. In that case, I'd recommend Heavy Metal for Heavy Slam spammage. There aren't very many things in-game that can kill a Pokemon in one hit anyways. --- I claim to be the official Porygon2 of the Pokemon Black 2 board. Scientist MileRun: WaRy associate