The magic of the iPad (if you want to believe in it) is that the iPad itself kind of fades away, so it's just you and whatever you're doing on it, whether it's making music in GarageBand, playing a movie, reading a book or scanning photos. Turns out, in most situations, the iPad disappears a lot more readily when the border is black, empty space. Whenever you're watching Netflix or slaughtering a dude in Infinity Blade or flipping through photos with the Getty app on the white iPad, you're always aware of the fact that whatever you're doing is framed by a huge white border. It pulls you out of the moment, the software, the movie.
The only exception—kinda—is reading books or reading straight black-text-on-white-background websites , where the white background more readily blends in with the frame of the iPad. Oh, and at least on our white iPad 2, the colour temperature screen is definitely cooler (bluer) than our original iPad or our other iPad 2. We're not sure yet if it's particular to our unit, but we're also hesitant to think that Apple is making the white iPad screens cooler to compensate for the white border. (For a moment we thought it was an optical illusion, but the cooler screen persisted after putting the white iPad in a black case.)
It's hard to really capture the feeling on camera—it's something you really should see and evaluate in person—but hopefully you're getting a sense of what we're talking about. Just know that whichever colour you choose, it's going to literally affect the way you perceive things on your iPad.