Marvel dominated in the 80s and early 90s, but today I would but them roughly even with each other. During the rought part of the 80s, DC basicly survived on the loyalty of buyers to Superman and Batman.
DCs catching up has more to do with Marvel going down hill then the improvements in DCs comics. Since their peak in the 80s, the quality of Marvels writing has gone down. The stories are more politically correct, more black/white and the stories more shallow then before.
Just compare the XMen's two great villians, Magento and Apocalypse. Magneto is a villian, but if you know his background you can understand why he feels the way he does. Much of his motivation is a desire to protect the mutants that is so irrationally strong that he is willing to destroy the rest of humanity to do it. He is designed such that you generally approve of his goals, if not his methods.
He also generally goes about achiving these goals in a manner that has at least some chance of working. From trying to build a haven for mutants away from humanity to training his own teams of mutants, you can see how his actions are aimed at achiving his goals.
Apocalypse on the other hand seeks to survive, gather power and rule the planet. Why? Because he can, he really has no deeper motivations. He really is nothing more then a bandit that has stumbled across power sufficent to reshape the world around him.
Luckily for humanity, he is fairly stupid though. If he used his vast power and technology in a sensible manner he would have taken over the planet long ago. Instead he regularly embarks on inane plans that are designed more to trigger stories then achive his goals.
Jay