Arguing over which superheroes would win in a fight is a time-honored tradition among fans. Once solely the realm of comics, there have been enough superhero films that fans can argue over which cinematic teams would triumph, with a recent argument on Twitter pitting Fox's X-Men against the Marvel Cinematic Universe's Avengers.
While evidence was offered from a number of sources, include Marvel comic books, the image accompanying the tweet that kicked off the debate provides a specific match-up: the X-Men from X2: X-Men United and the Avengers from Avengers: Age of Ultron. It's a good one, given that it provides a fair baseline of experience between the two teams.
The Avengers Have a Hulk (and a God)
The Avengers have a definite edge in raw strength, especially because Colossus isn't an X-Man in X2. Wolverine likely would be neutralized by the Hulk in a head-to-head battle, taking out the mutant team's best fighter. Storm's powerset is similar to Thor's; she wouldn't be a pushover for him, but she also wouldn't be able to overcome someone who can defeat the Destroyer solo.
The X-Men Have Ruthless Allies in Magneto and Mystique
The X-Men's X2 roster introduces some wildcards, considering the heroes teamed up with two enemies, Magneto and Mystique, against a greater threat in William Stryker. The Avengers would be a similar unifying force for the mutants.
Magneto could neutralize the weapons of Captain America, Black Widow and Hawkeye with a wave of his hands. Iron Man's armor would take more effort, but could still be rendered useless. While Mystique can't copy skills or powers, impersonation would be enough to throw a wrench into the Avengers' plans. She could disrupt the team by posing as Captain America, or else turn the Hulk against his teammates by mimicking Black Widow.
Captain America Can Do This All Day
Cyclops never lived up to his reputation as a great leader in the Fox films. He instead took a back seat to Wolverine in X-Men, and spent most of X2 under Stryker's control. The X-Men succeeded as much despite Cyclops' presence as they did because of it. Even his optic blasts became a liability once enemies figured out he needed his visor to control them, something the Avengers would deduce quickly.
Captain America proved himself as a leader multiple times. Even as early in his Avengers career as Age of Ultron, he'd already led the team to victory against Loki and his Chitauri invasion, and teamed up with Black Widow to bring down Hydra. With the tenacity he's had since he was a skinny kid in Brooklyn, Steve Rogers might be able to will the Avengers to victory.
The Avengers Have No Defense Professor Xavier or Jean Grey's Telepathy
The X-Men's greatest advantage is in the mind. Given that the Avengers haven't faced any omega-level telepaths in the MCU, Earth's Mightiest Heroes would have no defenses for Jean Grey and Professor Xavier's psychic abilities. While Iron Man has shown some resistance to mind control, the rest of the team hasn't. Xavier and Grey could turn the Hulk against his allies in the same way that Loki did, but it's more likely they would cause him to revert to Bruce Banner. Either way, there's a good chance they could take him off the board.
Wanda Maximoff & Vision Are Powerful Yet Inexperienced in Age of Ultron
Wanda Maximoff is cited as a trump card by fans backing the Avengers in the online debate. After all, Wanda was already formidable in Age of Ultron, in which she used her powers to sow dissension in the Avengers before ultimately joining the team. Wanda has one thing working against her: She's inexperienced in Age of Ultron, and not yet the reality-warping force of nature that she became in WandaVision. While she could cause serious damage, the X-Men can deal with her, especially if her power draws the Phoenix out of Jean Grey.
Wanda's future husband might cause more trouble. While Vision is also inexperienced, being a functional newborn, his myriad powers make him more than a match for anyone the X-Men have to offer. Unless Professor Xavier is able to engage him in a thought experiment, Vision could take most of the X-Men by himself.
Who Would Win in the Battle of X2's X-Men vs. Age of Ultron's Avengers
As impressive as the Avengers are, the X-Men have a few trump cards that would exploit the weaknesses of the MCU's heroes. The inexperience of their most powerful new members would be a liability, especially against the X-Men's ruthless allies like Magneto and Mystique. There's also the likelihood that Jean Grey's Phoenix powers would manifest when things are at their worst for the X-Men.