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I originally wanted a white one but once I found out that the coloured plastic around the Home button was black, I bought a black one.

White back + black front != white iPhone.
 
i have had a white 3g, and now a black 3gs. I feel the white looks better over time, less fingerprints are shown on the back and all that. I do enjoy the black however, since i have a gray/silver case on it, and the black gives it a very macbook pro style to it with the black/gray colors.
 
I got the white one to match my MacBook. Kept it naked for a couple of week, then got a Switcheasy Rebel case in white, with an extra red spine. Everyone calls my iPhone the "Peppermint" cause it looks like a star mint, it does get comments. And when I dropped it a couple of weeks ago taking it out of my pocket, it landed on a corner, and no damage thanks to the case. The back of the case is all scratched up, but the phone itself is perfect under it.

Chef Jay
 
Initally, I purchased the black 8GB 3G when I switched to AT&T. However, as my music and apps piled up on my phone, I went on eBay and purchased a 16GB white. I really prefer the white over the black. With the white, you don't see the surface scratches on the back of the phone (I don't use any protection on the phone at all). With the black iPhone, I would see the swirl marks and the light surface scratches pretty easily. Black is definitely a sexy color, but I definitely prefer white. It also helps that my girlfriend has a black iPhone, so we don't get our phones mixed up when they're side-by-side.
 
The black one is simply that - black with chrome highlights. The white makes the bezel less prominent and gives it a more varied look. Also i havent had a white iGadget since my 1st Gen iPod classic.

And I think my white one looks like a chocolate bar, which is nice. :p

...Of course colour is all subjective!
 
Still liked the original iPhone color scheme best--but of the current options, I prefer the look of the white one--at least until the hairline cracks start showing up.
 
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