1. The only correlation that i can think of between Magneto and Storm about their will power would be that fact that electricity helps them both resist telepathy.
2.IMO? it has to be Storm who has stronger telepathic resistance and stronger will-power, mainly because of the telepathic big-shots that she's resisted throughout the MU (as well as the feats that's she's accomplished with her will-power) and those that she's even kept harbored inside her mind,
I haven't really seen much feats about Magneto (if any) concerning telepathic resistance but im familiar with the fights he's had with Charles but still im not going to say until i see the extent of his resistance,
As Storm and Magneto have both had rough child-hoods im pretty sure that's where their will-power comes from, not to mention Storm and her claustrophobia, Magneto and the human-race etc.,