As Stakatacool said, Heavy Metal is better, mainly because weight-based moves like Grass Knot or Sky Drop are not very effective on Steel types, and the only thing you’ll have to fear is Low Kick, and still then, Steel types usually have high Defense.
And with Heavy Slam, its base power goes up to 120, which, with STAB, adds up to a total of 180 base power. You can do tremendous damage with Metal Coat, if your Steel type has high Attack, both Ingame and competitively.
Even then, not many Pokémon use Low Kick, as it’s not really viable to rely on the opponent’s weight to deal damage, so most Pokémons will opt for more powerful moves like Hammer Arm, Close Combat or Superpower.