I've been using scizor and steelix quite heavily now and noticed a few things...
Btw this is my moveset :
Scizor:
Slash
Aerial Ace
Swords dance
Toxic
Steelix:
Earthquake
Iron tail
Toxic
Iron defense
Scizor, especially with an adamant nature and if u actually ev train him for attack... He can practically one hit anyone... My scizor is lvl 80 and with swords dance and slash I could get through the entire elite four just using one scizor. However steelix, his base defense is OUed... But for a huge metal snake... His attack just isn't there... Plus, Scizor has 9 resistances and a immunity which is damn OP. So I would personally recommend scizor over steelix.