Here is an article about the Xbox 360 VS PS3 graphics comparison by gamespot .
It is apparent that few of Multi platform games look essentially identical on both consoles whereas the other are better on Xbox 360 due to better shadows , lighting , higher resolution or more stable frame rate .
Games included in this contest :
Fallout 3 – looks better on X360
Soul caliber IV – looks identical on both platforms
Dead space – looks slightly better on 360
Star wars : The Force unleashed - is more detailed and with higher resolution on Xbox 360
Call of Duty world at war – looks slightly better on Xbox 360
Madden NFL : 09 - looks identical on both platforms
Mortal Kombat VS DC universe – looks better on Xbox 360 due to better lighting and higher resolution