SGX, Fighting only does 2x damage because it's Steel/Ground, not Steel/Rock like Aggron.
In this debate, I completely side with Scizor. Steelix is ok, but I heavily prefer Scizor. Like Ben said, he makes a
GREAT Baton Pass combo from Ninjask. Also, as n_d said, Steelix's sole purpose is a wall against basically anything except Fighting- and Ground-type attacks. Fire-types could mangle both Scizor and Steelix, so that's no excuse to say that Steelix is better.