iPhone 4S > iPhone 4
My dad's iPhone 4 screen (and my sister's iPod touch 4G) screens almost look interlaced, while my iPhone 4S's is flawless with no interlacing visible. The 4 screen almost has a similar contrast to my old iPod touch 2G screen where all the whites are very vibrant and it almost looks cheap, while the 4S has a more warm screen to it. I personally think they upgraded the screens, but i am not entirely sure if they did.