White. Black looks great when it's clean, but the instant you get fingerprints or scratches on it, it'll look like crap. (That's the problem with black gloss.) White will look good even when it's been beaten up fairly thoroughly.
I ordered a 4 GB white nano early today... as with my iPod, it will be living on a clean white T-shirt until I get some sort of protective case for it.
EDIT: I have a PSP, and it's impossible to keep clean... not to mention the black car I used to own, which would get dirty between the car wash and my house. Black gloss is just not worth it unless it's stored in a hermetically-sealed environment.