Well, this is kind of awkward, but I voted that they are equally pretty. Kairi was a little bratty-looking bigheaded teen who had no clue what went on in the world in KH1... then there was a sudden change in my opinion as I saw more and more of her in KH2... she's tolerable to look at now. Namine was pretty in KH:CoM... well, her PS2 adaptation didn't go very well... she's tolerable in cutscenes but in the FMV's... It's not as good...
Then again, I think I'm more partial towards Namine, since her personality is nice. I'm not saying anything about Kairi, she's great in her own way, but I just like Namine more.