The Avengers consist of a billionaire scientist, a Norse god, and a Hulk who all, well, avenge—and all got their powers and skills a variety of different ways. But the X-Men are mutants, just people born with extraordinary and powerful gifts. Living in a world that hates and fears them, the X-Men swear to make the world a better place by protecting both human and mutantkind against forces of evil that stand in the way of peace.
Unfortunately, X-Men's movie franchise is still part of 20th Century Fox, while the Avengers are owned by Disney—so it'll be awhile before we get to see the teams face off, even after the sale.
In the comics, Quicksilver and Scarlet Witch were part of the X-Men, the Brotherhood of Mutants, and the Avengers. So, both Marvel and Fox decided to feature Quicksilver in the movies.
Let's be honest: X-Men's movie franchise features a much better, faster and cooler Quicksilver. We won't spoil much, but "Age of Ultron" proved that Quicksilver was the weakest link in the MCU. And that's saying a lot since the Avengers count Hawkeye among their ranks.
But the "X-Men" variant of Quicksilver has not one, but two standout scenes. Quicksilver is better off as a member of the X-Men and definitely would kick some Avenger butt.
On one hand, Wolverine can out-smart the Hulk. But on the other hand, Wolverine is a puny mutant compared to Hulk. Hulk could easily smash Logan into a pulp so small and nasty that not even Wolverine's superhuman healing powers can save him.