Don't really see how this is a debate, the most popular property in pop culture is almost always going to be the one who made the most recent big splash. X-men haven't had a big movie since X-2 and while first class was a quality movie, it never really resonated with the general public. The Avengers just shattered box office records, and just about everyone is talking about it in some fashion. Until the people at FOX get their act together and get the X franchise moving again, or failing that Marvel pulls another amazing X cartoon out of their butt the the Avengers are going to dominate the Mainstream audience.
Now if you want to talk comic audiences that's a whole different thread.