Extremely tough battle. I'm disappointed that they didn't give more time to show off Legion's power. I actually think Legion is more powerful and would give him the win.
Completely agree. I personally feel he was always intended to be a powerhouse, purely because it would seem likely that the writers would like to have Xavier's son be one powerful mofo. And he's said to possess ten times the psionic power that Xavier does, which is pretty intense. He's simply limited due to his multiple personalities and how each can only access one of his powers.
Powerwise, X-Man appears to have been one of the greatest of all the mutants, but I'd seriously question his control. With Legion not being limited like he is in the comics, I'd say he could likely take down Nate.
Ah, but he fixed that problem. In Legion Quest, it was stated he healed his mind and was able to access all of his abilities. He was even able to create a new one...time travelling.
In a healed mind, Legion display of power just skyrocketed. His potential was also clocked at around Dark Phoenix level. Plus he seems to attain powers of those he killed. Personally if Legion would have continued he would have ended being the most powerful Alpha mutant in Marvel.
However, at his best his performance is shadowed by Shaman X-Man. Who can now wield his Psi/Tp/Tk more skillfully thanks to the merging of the alternate Nate Grey.