Odin hung himself from a tree for nine days and nine nights, and pierced himself with a spear. He also ripped out his own eye.

It seems they’re both equally “metal”.

Why did he do that to himself?

He hanged himself to obtain the secret of the runes, so basically to learn magic. His eye was given away for a sip from Mimir’s well, the Well of Urd, at the roots of Yggdrasil, which gave Odin cosmic knowledge and wisdom.