If it's full roster, X-men (200 or so I'm guessing this is just the current roster, the dead are not included??) win this. the Legion of superheroes has more numbers, but X-men are far more powerful. and Multi-man could create countless dups. You have omega level telepaths, and telekinetics, who would just destroy them horribly. Saturn girl would be crushed by the Cuckoos and Emma, then you have Charles, Psylocke, Jean, Maddie, Quentin, Cassandra Nova, Cable, X-man, (in his prime) Rachel ,& M i guess could help but she's not on their level. Legion alone will be a hand full especially if he start off reality warping, not even mentioning his other powers, more that half of that roster won't even exist. Then you have Iceman, Storm, Dust, Cyclops, Magma, Polaris, Magneto, Meggan, Havok, Banshee, Sunfire, Thunderbird, taking out a wide range of people. Not to mention if anyone dies on the X-men Layla miller can bring them back to life.
The problem with the legion is that yes all the members have powers, but generally they are not that powerful and only have a singular talent.