Guys, im reading some guides, and they recommend a magnezone to trap steel types your opponent is using, together with secret power.

Theres a guy in platinum that tells you the type of secret power your pokemon would have if he learned secret power, and im somewhat confused because the guides recommend different secret powers.

Some say fire, some say ice, and one said grass.

Fire is strong against grass, ice, bug, and steel, so it makes sense since the idea is to use magnezone to kill/trap steel types like scizor
Grass is strong against water, ground, rock and ghost so I dont see why it is recommended for a "steel killer" other than it is strong against rock and ground, which would give my electric attacks trouble.
Ice is strong against grass, ground, flying and dragon. I assume its there to help against grass and ground, same as grass would be there to help against rock and ground.

And personally, I wonder, why not fighting? Fighting is strong against normal, ice, rock, dark and steel, and I see lots of people using rock, dark and steel these days.

I went to ironworks and burnt about 50 pokeballs to catch enough magnemite to say I could sell them by the dozen, so I´m just trying to decide on which type of secret power magnemite to keep (and train into magnezone).

And yes, I know I should try to get one with an adamant personality, but looking for two random attributes (adamant personality and secret power (ice/grass/fire/figthing) would be quite the task.

Which type of secret power should I go with?