Boy, talk about a tough choice!
Both women have similar body types: slender, long legs, and huge natural boobs (Rachel recently got a boob job, but prior to that she was naturally busty). I believe that Rachel is the more classically beautiful of the two, and while she seldom does hardcore (I saw a video long time ago where her husband was fucking her in a sex swing, but that's the only time I've seen her do actual intercourse) her masturbation and blowjob scenes never fail to turn me on.
Kelly, on the other hand, seems like she'd be a real firecracker in the sack. Her Pornfidelity scenes are raw and uninhibited, and she's also willing to participate in threesomes with other women. She also recently did anal for the first time on camera.
Sex with Rachel I believe would be more slow and sensual yet still hot as hell, while with Kelly it would be wild, energetic, porno-style sex. If Kelly wanted to bring along one of her Porn star friends to have a threesome, she'd win in a heartbeat. But if it were one-on-one sex I'd probably go with Rachel.