Two (girl)friends, how do i turn one down but go out with the other?

im in a group of friends where two of the girls like me. I really want to enter into a relationship with one of them. The other keeps trying to push for a relationship with me. I don't have a real physical attraction to her, but get on well with her. How do I turn her down without hurting her and losing her from the group. I'd like to keep her as friend. I'm concerned she might turn against me and the other girl.