When you go black, you'll never go back.
I currently own a first generation iPad and will most likely buy the third generation iPad and, to be honest, I'm still in doubt.
The black border is more natural, but I also found that the white iPad 2 feels fresh and makes it seem as if the screen has a higher contrast.
When you, for example, watch a movie with letterboxes, I found that you will see that the black border is of a deeper black than the screen itself. However, if you were using a white iPad, than the letterbox would appear to be really black (because you can't compare it to a black border).