The Xbox 360 is a more balanced architecture and therefore can produce better results on general purpose game engine/designs. PS3 exclusives tend to be coded and designed to maximise the advantages of the PS3 architecture whilst minimising the flaws and they have the advantage of not being compared to an equivalent Xbox 360 title.
Where I notice people beat themselves over the head needlessly is when they begin with the assumption that the PS3 games ought to play better than the Xbox 360 because of the MOAR power of the PS3 and when they assume that the Uncharted streaming engine can be ported over 'Oh you naughty open world developers, If Uncharted 2 doesn't need an install with a closed linear game that you cannot backtrack you must be stoopid and lazy because Naughty Dog can do it'.
So in short: Xbox 360 can be easily tasked to a wide variety of game conditions/designs and the PS3 cannot. Also the Xbox 360 sells a lot of software so you've got to make sure your best bet for making a profit actually runs well.
