While I agree that the Avengers are bigger in pop culture right now than the X-Men it's mainly because of the movie the X-Men imo is the team equivalent to Superman, Batman and Spider-Man. What I mean by that is they don't need to have a heavily marketed movie to stay culturally relevant they'll always be popular. Example, my sister who knows pretty much nothing about comic books knows who the major X-Men are, she knows Wolverine, Storm, Rogue, Cyclops she even knows who Jubilee is.
She also knows some villains, she knows who Magneto, Mystique and Juggernaut are. Whereas I don't know how but just yesterday she heard about how much the Avengers made opening weekend and asked me "Who are they? What's that movie about?" I had to tell her they were a superhero team. Whereas she just knows who the X-Men are. In fact I can only guarantee the only Avenger she knows is Hulk, she might know Iron Man, and Cap but not the others I know my sister doesn't represent everyone but I like to use her to see how the general public views comic book characters. In the NY Daily News they called Cap Captain Planet, so I'm not sure of how much bigger they really are over the X-Men.