Fear Factory are more veterans of the metal scene, but I feel like Devildriver would probably bring in a larger crowd at this point. Only seen them once, and the crowd went nuts. That was opening for Arch Enemy though, so who knows how many would have came if it were just DD. Never seen FF live, but I just don't see them doing well as a headliner of a large package tour at this point, but I could be wrong, since I don;t really follow FF that closely.
5/15 Full of Hell/The Body
5/19 Water Torture