I'm swaying towards Legion unless persuaded otherwise. He doesn't have such an obvious weakness as Proteus' metal allergy, and I believe him to be the better telepath.
Legion has unexplored Telepathic/Telekinetic/Pyrokinetic abilities plus he has time-travel for all we know he could of had other abilities. Had he lived long enough, he could have ended up as the most powerful mutant to exist, or more likely the most powerful telepath to ever exist.
I'm going to say Legion. He just had too much power. I mean, he learned how to time travel using his own power. What other mutant telepath has done that?
Proteus would just posses Legions body, burn it out, then they both die.
__________________ Huey: The truth hurts, Jazmine. The world is a sad lonely place and no one gets anything for free, you know what? One day you and everyone you know is gonna die.
Proteus has a glaringly obvious weakness, and thats lack of experience or potential, I'm sure Legion could exploit this or use it to his advantage............