I think Terrax, who controls earth, is at a more unique disadvantage against Magneto because of that - Magneto could pretty much counter everything that Terrax does. If Terrax were more in touch with the essence of Power Cosmic, he could win easily. As it stands, he comes across as more of a super Moses Magnum.
Classic Magneto takes 5/10
Current Magneto stomps 8/10
Terrax's axe could cut through anything Magneto threw at him, and I'm fairly sure Magneto wouldn't be able to use his powers to pull the axe from his hand because it's a cosmic weapon...I'm sure mere magnetism isn't going to be one of it's short comings.
Aside from being able to cut through anything Magneto throws at him, I'm quite sure his body could take whatever Magneto could dish out.
Terrax