If we're looking at it from a real world point of view. The Avengers went back and forth with the Fantastic Four during the 60's and 70's at Marvel, while the X-Men struggled and even went into reprints for five years in the first half of the 70's. They didn't start outselling both books until 1981 and that really lasted until the Bendis run. Likewise over at DC, the JLA was the top selling book until 1981, when it was replaced by The New Teen Titans and Legion Of Superheros until 1987, when the new Justice League series launched. If we're looking within the universes they occupy, then it is a different matter.