When it comes to FPS, PCs definitely win this battle. You have more control over your aim because, technically, you get to point at where you want to shoot and you get the job done. Counter-Strike is a nice example of how well shooters work out on PC.

EDIT: however this line is starting to get kind of blurred because look at consoles now, you have the technology to use mouse and keyboard if the game supported it. Also you have motion control technology. Even with that knowledge, PCs are still the dominant systems for shooters.