My money (and vote) goes to Yomiko, because although she only controls paper and Sakura has the ability to call upon some much greater powers depending on her card, she only has a finite number of cards and a limited amount of power with which to use them. She'd get exhausted in battle fairly quickly. Yomiko, on the other hand, while she only has paper, also has the ability to change it at her will, making it something much more dynamic than an element by itself.
But, I think Sakura's prettier. Especially in Tsubasa Reservoir Chronicle, heh. Even so, I think Yomiko's the better of the two.
Still think Yomiko is a bit of a selfish character (my book, wanting to be the main character in the book, whining when she doesn't get her own way etc.). Sakura's much cuter and could take her with the flame card easy!