I massively dispute this, Halo players have a completely different style of aim compared to say, a counterstrike player. You will often find halo players using movement to 'align' the crosshair with the enemy, neglecting the right thumbstick, which is responsible for crosshair movement alone (aim). In comparison a counterstrike gamer does not go about his business this way, instead there is a massive focus on pre-aiming, keeping the crosshair in a useful position and when it actually comes to hitting that guy in the head, the hand and the mouse are fully responsible.
Two different playstyles for me, and although I agree there are hugely talented halo players out there I do believe that pure potential lies with the PC gamer purely for the fact that a mouse/mousepad is easier to use, much much more versatile and furthermore your using your hand, and not your thumb.