I don't agree. I am currently leveling an Arms Warrior with the heirloom 2h weap. Mortal Strike is incredibly OP in the lower levels - I usually just charge and then Mortal Strike + Heroic Strike and they are dead. I don't even bother to rend

I have pretty much zero downtime, as the mobs don't even have time to hit me. So if you have a proper twohander - go for arms, defo.