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Iron Head is a better choice, imo. It does more consistent damage and has that nice flinch chance. Aggron has a number of ways to end up faster, whether going for a T-Wave Parahax set or a Rock Polish/Autotomize sweeper set.
The damage of Heavy Slam isn't worth giving up your ability (and getting annihilated by Low Kick or Grass Gnot), and Heavy Slam isn't worth using without Heavy Metal. Just do Iron Head. Rock Head for Head Smash is a good call, even if it looses it as a Mega.
If you are going to Mega Aggron, do it sooner rather than later. Aggron appreciates ditching that Rock typing, and can't patch up any mistakes.
