Nate Grey is a powerful psionic (telepathy and telekinesis), while Legion with his three personalities have telekinesis (Jack), telepathy (Jemail), and pryokinesis (Cyndi).
I'm unsure how effective Nate's telepathy may have with Legion's personalities (yet Shadow King was able to subdue him before), but by sheer power, Nate is superior and would probably win. Legion has shown some fear in using "Jack," so if he's as fearsome as Legion may make him, depending on which personality is the most dominant during the circumstances, Legion may not be a push over.
Nate Grey at full potential could take him, his power would just overwhelm legion. he does have three personalities...hes supposed to be some sort of troubled child of prof X with immense power. his powers are tha same as nate but hes got making fire ability, which doesn't out-weight nates superiority in psionics.
Lol, havok...well yeah i am a definite fan so i have knowledge of nate, but i also have knowledge of legion and i do not believe him to be on nates level....hes one of a few who are very close though.