Not much in it as far as looks. Colours seem a bit more vibrant in Pcars but that is just settings.
Both very very realistic. Too realistic really to the point of being sterile. give us more dirt!
Sound? well, shitty work computer speakers wont do either justice. I've never heard a SLS AMG GT3 in real life so I cant comment, but of cause, I'll give an opinion. AC sounds better.
Driving? don't have Pcars. and i'm a AC fan boi.