Let' compare them:
Hammer Arm's power is 100, with 90 accuracy, it has 10 PP and lowers speed after a use. It has STAB with Chestnaught so its power become 150.
Effect: Lowers Speed
Close Combat's power is 120, 180 with STAB and has 100 % of accuracy, 5 PP and lowers both defensive stats.
Level: I don't know
Effect: Lowers Defense and Special Defense
Chestnaught has a great defense stat teamed with a pretty special defense. It has bulk. BUT, it is severely harmed by its X4 Weakness to Flying. This means Close Combat will weaken it, but must hit very hard, and if luckily enough it's a OHKO, you're safe. But with Chestnaught's Speed, I already see any Pokemon spam Air Slash. I think that your final choice would be Hammer Arm, because it can be used strategically with Gardevoire's low speed in a Trick Room set ; you would have a hard-hitting tank in Chestaught !
Hammer Arm > Close Combat
Hope this help !