I agree with Eleven, even though i can't say i know a lot about MM both characters seem to be at least comparable in terms of psychic powers, however, MM might have the edge in terms of experience and abilities.
About that "nobody over the age of 12 loves the JLA", its funny how every comic book store or convention i ever went to always say that DC aims at an older audience, while Marvel, even though their trying to fix their Fisher Price image with the new Marvel Knights and MAX series, which are excellent btw, aim at a younger audience. I know very few comic book fans over the age of 21 who are huge Marvel fans, they either like both or prefer DC.
I myself am 26 and am not a fan boy of either company, im a fan of certain characters. Im glad though that Marvel started those two "more mature" series i mentioned above, through them i can revisit old favorites of mine that i just left behind gradually as i grew older.