I prefer DC over Marvel, because DC maintains a more consistent, higher level of quality in their books on average.
Face it - every month, Marvel publishes some real crap. For every She-Hulk, Runaways, Captain Marvel, or Supreme Power there's a dozen comics like Iron Fist, Alpha Flight, Emma Frost, Exiles, Human Torch, Academy X, Venom, Weapon X, etc., etc. I'm sure some of those series have their fans, but at best the books are mediocre. Marvel literally floods the market with this kind of rack-wasting garbage. They also milk everything that sells to the point they can't get another dollar out of it. It's the mentality that killed characters like The Punisher and Ghost Rider, and imprints like 2099 and Midnight Sons in the 90's, except Marvel does this month-in and month-out. Not content to have a couple Spider-Man books, a few X-Men books, etc. - Marvel's got a to have a half-dozen imprints like MAX, Marvel Knights, Ultimates, Marvel Age, etc., etc. - and crank out Spidey and X-Men books for EACH ONE. I don't see DC launching VERTIGO-Superman or WILDSTORM-Batman knock-offs. They have a little more respect for their characters and the imprints they've established.
Let's look at those imprints, shall we? Veritgo kicks MAX's ass ten ways to Sunday. You practically can't find a Vertigo book that isn't worth reading, whereas Supreme Power is virtually the only MAX book worth the paper it's printed on. Maybe Punisher, now that they've switched it to MAX (so they can milk you for another #1 - thanks, Marvel!). Besides Vertigo, DC has the Wildstorm stuff - which I can take or leave, personally - but it's still a better line overall than MAX. Then there's Alan Moore's ABC subsection, which produces some brilliant comics - better than the majority of Marvel's mainstream core titles, IMO.
Another thing DC hasn't done (yet) is re-launch their books over and over. Superman, Wonder Woman, Flash - those series were actually allowed to reach #200 again. It'll be a cold day in hell before Marvel ever allows a title they've launched in the last ten years to make it that far. MUST HAVE ANOTHER #1!! CAN'T SURVIVE WITHOUT CHEAP WAY TO TEMPORARILY SPIKE SALES!! Try getting some quality creators on-board for a change, Marvel. But, no...that might actually cost money, and the House of Idea knows they can just slap "#1" on something and their loyal fans will buy it sight unseen.
DC is an innovator - Marvel is a follower. Adult-oriented comics? DC innovated them with books like Watchmen, Swamp Thing, Animal Man, and eventually Vertigo. Mini-series? DC invented them with World of Krypton back in '79. Trade paperbacks? DC started printing them first - they invented the format. Superheroes in general? Yup, DC again. Not to mention the creators DC discovered, who went on to popularity at Marvel - Morrison, McFarlane, Ennis, Millar, Johns, Milligan, Gaiman, Rucka, etc. It's a huge list.
The one thing I give Marvel credit for over DC is their willingness to publish trades - but again, it's almost to the point they'll print anything as a trade to make a buck. Quality really isn't the concern for Marvel - flood the shelves. DC pisses me off becuase they're so goddamn tentative about publishing trades. ALL of Hellblazer should be available in TPB. Sandman Mystery Theatre should be completely collected in trades. The Question, The Spectre (Ostrander & Mandrake), Suicide Squad, the rest of Hitman, ALL of Morrison's Doom Patrol - DC's got so much great stuff that people might actually discover if they'd print it in a ****ing trade. But they don't. Meanwhile, Marvel is cranking out the 9th collected edition of Extreme X-Men, or some similar birdcage lining.
Across the board, DC maintains a consistent quality level. You can pick up Birds of Prey, or Fallen Angel, or Richard Dragon, or Catwoman, or Aquaman, or Batgirl, or Gotham Central, or Green Arrow, or Hawkman - lots of titles that aren't Superman or Batman, and therefore, don't sell especially well - and the end result is it's usually a pretty good comic. Marvel can't even say that on a regular basis about Thor, Iron Man, or Hulk - let alone the stacks of lesser titles they see fit to print.