I am not refering only to the "gritty" realism but to the way animation, physics and effects are being displayied.
What impressed me so far with PS3\'s promises is the level of realism on physics and animation.
Although I dont see MGS4 in motion the poses and effects give an idea.
GOW is the most impressive XBOX360 game so far.It displays a level of detail probably surpassing the detail seen in these scans but the whole scene looks lifeless but extremely detailed.The same counts for other 360 games
My point is that the difference in art direction is probably a result from the difference in capabilities as well.
The tech demos showed at E3 werent aiming at visual detail anyways but to the level of detail in behaviour and physics.
So the emphasis was mostly on how natural/lifelike the games will feel.
Although Killzone,Motorstorm, and even Heavenly Sword(which had a less realistic art direction) probably didnt run real time on actual hardware emphasized on the level of natural feeling in interactivity, animation and behaviour.
I am not saying that 360 cant do any of these as if its a prooven fact but I am guessing from what I ve seen so far that this is what might make the PS3 stand out.