With every type of "model" site there has to be a line of progression, without this the site will eventually die.
Christina is going in the right direction, seeing her fluff would be nice and I think, in time, we will "see all"..
Nikki on the other hand isn't really going in any direction, she seems stuck in an endless NN cycle, cool if it were 5 years ago, but paying members in 2010 want to see some skin for their hard earned buck, at least full topless.
I still think both models are super hot, but I think Christina has the edge on site longevity over Nikki.