Tough call! Black seems to be better for watching videos because it blends into the aspect ratio black bars and it's less obtrusive. White seems to be better for reading and browsing because it blends into the white background behind the text and it makes the screen feel more spacious (at least for me).
I'm probably going to use my ipad for browsing more than video, so I'll probably go with white. And for some reason, the black border makes the screen feel more cramped and restrictive in general usage...but that could just be me.
I've been spending a lot of time playing with demo's of both and trying to pick the best color for everyday usage...but bottom line is, I wouldn't complain with either 🙂