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In terms of look, 4.

But in terms of feel, the 3GS by far. The curved plastic feels way better in the hand compared to the sharp edges+coldness of the 4's metal band and glass.

Threw a bumper on the 4 and that's much nicer.
 
Still fond of my 3GS in white, if they came out with that design with the iP4 features it would be a no brainer, however I like my iP4 as well especially the speed and screen. One thing I can not say has been a drastic change is the battery life, I feel the way I use my phone it is equal to my 3GS, maybe just a tad more life then the 3GS.
 
I loved the original iPhone, but between the 3G/S and 4? No question.

The cheap plastic look and feel of the 3's was awful, whereas the iPhone 4 is simple and solid feeling (even if it is prone to cracking on the pavement).
 
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