I don't think there are any "stats" on which a girl is more likely to prefer, and even then, your girlfriend isn't the same as the majority of girls (or else she wouldn't be your girlfriend, right? 😛 )
You're also looking at a $200 price difference, as well as a larger drive.
Does she need the larger drive?
Base it on the specs she needs before the colors, IMO.
But, even still, I bet she likes the white one.
Or, you could ask her about the iPods, which she likes - black or white. That wouldn't ruin the surprise, and would be a good base to judge from.