Originally Posted by Anubis
I mean he was an Avenger for years. He's never been an X-Man. He was in X-Factor for a little bit, but most of his 40+ years has been as an Avenger.
He and his sister just kinda happened to have been Mutants.
Quicksilver and Scarlet Witch were mainstays of the Avengers; core of the corps. Their 80s retcon into Magneto's brood led to some major story arcs, but neither Maximoff could be accused of being "X-Men"; or even Brotherhood, save for a few initial issues before joining the Avengers.
When you compare the number of times the characters appeared in the Avengers, vs. any X-titles, it's not even remotely close. Avengers all the way.