Here's the issue, sony required that the ps3 outperform the xbox 360 from the very start to justify the price range, because of this their development teams were given extremely large budgets and very long development times. Remeber the ps3 drought at the beginning of the generation? If sony failed to maintain a graphics quality at launch that could compete with what the 360 was offering then much of the hype generated by their cgi bullshot e3 trailers would disappear along with sales. Games released early failed to compete with the 360 titles because of restraints created by the platform.
MS on the otherhand was focused on trying to deliver a consistent experience on their console, for users and developers, especially with rrod fiasco. Along with the better development tools it meant that developers could be lazy with 360 development and not push the hardware as much as sony first party was forced to do. Microsoft pushed out games faster and on smaller budgets, making sure developers were happy and costs were dropping, and games were being released consistently, letting the quality improve over time.
In my opinion, the ps3 has began to stagnate with regards to the graphical fidelity, killzone 2 and uncharted 2 looks as good as anything I've seen released on the ps3, where as games on the 360 i continue to notice vast improvments.