Quoted:
At first glance the Avengers are more powerful however the X men have variety.
Professor X can calm the Hulk and cause him to revert to Bruce Banner
Magneto OWNS Iron man and Hawkeye.
Wolverine Beast and Cyclops can handle Iron Man and the Shield agents and probably Cap.
Jean Gray can probably handle Thor.
Plus they have Ice Boy and depending on the team makeup, Nightcrawler or his mother Mystique, who would wreck any shield agents.
So the X men could triple team Thor if X can handle Hulk.
If the Avengers have Ant Man and Vision it gets dicey though.
Sabretooth has been an X man once or twice he is nasty to fight. Also banshee is destructive as hell.
Plus rogue effectively has the powers of ms marvel, so she is a pretty tough cookie.
You could also throw in cannonball, sunfire and you also have forge who is effectively a better inventor then tony stark.