CAUTION: NERD STUFF
I'm a big fan of the Batman movies, but the brilliance of Avengers is that it feels more like a Marvel comic book to me than the Batman movies do. The Dark Knight is more in the vein of The Dark Knight Returns series, which to me is one of the greatest graphic novels ever written, but not so much as a classic serial comic book.
While Nolan's movies are fascinating with some of the most gut-wrenching villains around and great commentary on the hero, I think Whedon brought the fun back to comic book movies while not sacrificing character study.
Long story short, I think I actually prefer Whedon's treatment over Nolan's, but thank goodness we live in a time where we have access to both.