That depends on your team. Both of them have different types and movepools, though I would select Haxorus for its high attack and good movepool; Outrage and Dragon Claw are both good options. It's speed is also very acceptable, and has access to D-Dance. On the other hand, Gallade is dual typed, but does not have an excellent movepool and is not Defensivly bulky, though it should withstand a Special attack.
Purely your choice, it all depends on the team you are building around your Pokemon.