Definetely Keira. I can understand why people think Natalie's hot, but she just doesn't do anything for me. I'd still hit it if I had the chance, but if I'm getting a choice it's all about Knightley.
I almost prefer the big ones to look at, especially if they're kinda puffy. For use, it doesn't really matter as long as they're on a female.
I bet Charlotte Church likes it in the butt, too. I'd hit it.