Marvel for an admittedly shallow reason: I love Transformers, the comic for which was published by Marvel until 1994. When I collected them, I just got more used to seeing their superheroes in the ads, and I still feel like I "get" the Marvel universe more than DC.
Maybe it's because I also collected some of the 80's guest-star-of-the-week titles like Dazzler and Power Pack, but the Marvel universe always seemed to have an easy-going feel of community. Whenever I'd read any post-crisis DC book, there was just something stiff and unpleasant about it.
DC has consistently put out better cartoons though. Even the best X-men cartoons can't hold a candle to the Bruce Timm universe ("Batman the Animated Series" through "Justice League Unlimited"), or "Teen Titans". "The Batman" sucks though. Haven't seen "League of SuperHeroes" yet.
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No one in the world ever gets what they want,
and that is beautiful.
Everybody dies frustrated and sad,
and that is beautiful.
-TMBG