Between the Big Two I would say that DC is better with their usage of female characters. Marvel has a habit of sticking to a certain female or small group of females for each of their franchises; Spider-woman, Storm, Invisible Woman, Captain Marvel, Emma Frost,Black Widow, Psylocke, Red She-hulk (to an extent) are just of the few that have been used ad nauseum over the past couple of years even though Marvel still has a large category of unused female characters, but still even when those select few women were used they barely get the same panel+story+development time that their male counterparts are getting.
DC has a bit more balance with their female superheroes/villains/side characters. Just sticking for the new 52 for now you can see that there's been a lot of female characters with headlining solo or team books since new 52 started. Batgirl, Batwoman, Birds of Prey,Catwoman,Hawk and Dove, Justice League Dark, Legion Lost,Legion of Super-Heroes, Red Hood and the Outlaws, Red Lanterns, Justice League of America,Suicide Squad, Supergirl, Teen Titans, Voodoo, Katana, Wonder Woman, World’s Finest are just some of the few that showcased a variety of leading ladies in one way or the other.
DC's treatment of their characters is piss poor all the way around, but mostly their females suffer from terrible writing and the curse of fridging for some dude's manpain, however, at least DC tries to expand their character use.
Although, they've been getting a little better lately Marvel still will just stick to the same ole', same ole' character roster without thinking of giving someone else a chance to shine.
All that being said, I still prefer Top Cow over both the Big Two, lol. But Image as a whole has been the best to female characters these past few years, so I voted for them