Black looks extremely clean and everything is black. When watching movies and using apps that haven't been optimised for the larger screen, it would look better. It will also have a higher resale value than a blue one if you decide to sell it later on. Downside, shows fingerprints.
White/Blue is more fun and exciting, the white bezel won't show fingerprints but the white bezel makes the ipod look a bit messier because you can see the camera and the black outline of the screen.
I'm buying the black one because I have a white iPod Touch 4g and want to switch it up a bit. It really is up to personal preference though.