In the Washington Post, Hank Stuever asks the really big question (link courtesy Griff):

Marvel or DC?
Back when it mattered, you used to be certain. You would ally yourself and endlessly argue the merits in comic-book stores or at a convention at the airport Ramada. DC Comics, led by Superman, was for people who adored the fantasy, the Ubermensch triumphant. These readers loved skyscrapers and archvillains and sidekicks, billowing flags, unerring ethical strength.

Marvel, led by Spider-Man, was a place for the smart but troubled reader, the deeply weird. They loved the night, the underground, accidents in the lab. All that dialogue, so many thought balloons! The heroes always on some emotional ledge, and the hubris of it all — a grittiness that came with saving the world.
DC was about younger kids in back yards, wearing bath towel capes, leaping from treehouses.
Marvel was about older kids in basements, possibly stoned, deconstructing Thor.
DC invented places to go — Metropolis, Gotham City, Paradise Island.
In the Marvel universe, New York is New York, and it’s nothing but trouble.
DC: It was always the Fourth of July.
Marvel: It was always Halloween.
DC: Comic books are a wonderful escape.
Marvel: Comic books are a dark refuge.