First of all, these are tough choices as both mons function very efficiently in the same way.
Haxorus has a higher base speed than Rampardos, however, Rampardos' attack is higher.
Rampardos functions well with a choice scarf, but Haxorus can be banded
Haxorus has an excellent ability in mold breaker, but Rampardos has the same ability and a better one in Sheer Force, which makes its attacks even more powerful.
Haxorus can use STAB outrage, which has a base power of 120, however, Rampardos gets head smash, which is 30 points more powerful at BP 150.
So, in the end, I would have to say Rampardos due to its ridiculous attack and its versatility, as it can also be run with Mold Breaker over Sheer Force to set rocks.
Rampardos @ Choice Scarf
Ability: Sheer Force
Rock Slide/Iron Head