Without a doubt I'd pick Nikki. Christina has started showing more but she still looks stiff and very uncomfortable. Nikki appears far more confidant and seems to enjoy teasing. Christina always looks very self conscious and clueless about how to flirt.
IMO being sexy isn't just a matter of being naked. Even if Christina starts showing it all she won't be as sexy as Nikki.
I hope no one thinks I'm flaming Christina, I think she's a very pretty woman and she dose have a nicer ass then Nikki. However, between the two Nikki wins at being sexy every time.
Christina may have a inner freak but in all the pictures and videos I've seen it's been well hidden.