Can't resit on the PC / Console for FPS.
The difference can be quantified in that a mouse gives measureably (and demonstrably) more precision that a gamepad.
Now, I'm not saying better as an opinion or people shouldn't play FPS on consoles (after all I do).
All I'm saying is the best FPS player in the world on a mouse/keyboard should (and very probably would) be able to beat the best FPS player in the world on a gamepad simply because his input device gives him more precision and control.
That difference means a few millisecods more accuracy to make a kill of course.
So while I generally don't get into the whole PC / Console thing as I think its stupid - there is no denying the mouse/keyboard gives better input for FPS.