Actually, I disagree with Electrolyte. First of all, two Superpowers are stronger than two Brick Breaks (Superpower: 120 + 120*(2/3) = 200 BP, Brick Break: 75 + 75 = 150 BP). Also, the Choice Band negates the attack drop from Superpower, and a neutral Haxorus still hits hard. Being a Choiced Pokemon, it will have a hit-and-run playstyle, so Haxorus will switch out often, getting rid of any stat drops, and you'll want to hit as hard as you can before you switch out, similarly to Specs Latios.
So, TL;DR, Superpower over Brick Break on a Choice Band Haxorus.