I think the black one is better because you don't have to keep it as clean as the white one. The white one stains too and gets discolored which doesn't look nice. The black one is the better is definitely the better of the two.
White and I have a white iPhone and it does not stain because there is glass covering the color and even on the iPod there is glass covering it. Also I think white looks more sexy then black. Also on all my apple products that can be white are (eg. iPad iPhone iPod) and I don't clean it very often and it doesn't look dirty at all!