Having just mentioned Twilight, I thought I'd draw your attention to an excellent piece in the Guardian about it here.
Yes, Buffy (from Whedon's Buffy the Vampire Slayer) is a much better example for girls. Well, most of the time. Buffy wasn't perfect and if she were, we probably wouldn't have found her nearly so interesting. She made bad choices, allowed herself to be occasionally sacrificed to someone else's angst, and she had to die a couple of times.
While I do enjoy the first couple of books of the Twilight series, I'll add... I don't want to be Bella (that should go without saying). I do not see Bella as a good role model. Well... unless it be as a role model for melodramatic, teenage, romantic angst, in which case I think she outdoes even Juliet. Part of the delight of the first couple of books for me was how wrong Bella gets it.
In more intriguing news, I learned today from Meg Cabot's blog that there's a ferocious unicorns versus zombies debate. One of the leaders of the unicorn movement is Holly Black.