Haxorus. The thing looks ugly as sin in my view, like it ought to just fall flat on its face, but I'm no fan of Hydreigon's looks either.
Stats-wise they're about evenly meritous I think: Haxorus has monstrous attack, trollish just-below-100 speed, and mediocre everything else. Hyredigon has good SpA, also troll speed, enough Attack to go mixed, and better defences.
The big difference is boosting moves. Haxorus is blessed with both Dragon Dance and Swords Dance, so it can run sets with either of those, as well as Choiced sets. Hydreigon gets Work Up. Just not good enough.