I don't like either. Natalie turns me off by not being good actor - at least not as much as she is given credit for. When I watch something she is in, I'm constanly aware that she is 'acting', and that's not a good thing. I mean instead of the character, I'm watching her pretend to be the character. Plus she looks too much like my sister, so...no.
As for Keira, she always seems to look like she's about to bite the head off a cat. Still, if I had to choose, it would be her...after she washes the fur and cat blood out of her mouth...Although she comes across like a smug bitch.