Now it's well known that a lot of people prefer the Xbox 360's controller for games, especially FPS'. However, those who have played both consoles will more likely incline towards the PS3's controller for the analogs' precision. For the Xbox 360 controller's camera analog is a lot less precise, making precision tasks like headshots or sniping a chore to accomplish.
From my experience, I have played CoD4 on both consoles and have noticed that it's a lot harder to aim with the Xbox 360's controller. I tried reducing/increasing the sensitivity but it didn't change much, in fact, it was even a bit harder.
I also tried Halo 3 and I must say that aiming for headshots is a real pain.
All of the above are based on my experiences so don't take it as a fact.
Discuss from your experiences.