Well.. Sony has a bigger/better first party support ( IN MY OPINION ).
But they have more engines that are ..exclusive? compared to X360s first party (a fact I think..?)
This is Sonys 3rd gen while it's Microsofts 2nd. Sony has been in top with the PS2 and ..maybe PS1? Not sure about PS1.
Point is, they have more first party dudes with exclusive engines which makes us go O_O while a lot of the X360 games use the U3 engine (I think.. xD)
Im not really a expert on the engine area, but I think i'm partly right on what I said above. If not, please feel free to correct me ^^.