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