I have played with a mouse, and usually the aiming reticule remains steady in the center while the mouse turns you, this allows for fast turning based on how quickly you move the mouse, while you can still shoot. With a Wiimote the situation is different, turning depends on you pointing at a certain part of the screen, I would suspect that this would make turning and shooting more difficult on the Wii than on the PC or the gamepad. For certain games this works fine, such as RE4 since the console version doesn't allow you to strafe anyway and you always have to stand still before you shoot, but other games such as Unreal Tournament where turning has to be extremely fast and precise it would be a detriment. This, coupled with the fact that noone holds a wiimote as steadily as they do a mouse, and I think the difference between the mouse and Wiimote are much greater than you are assuming.
I don't think the Wiimote is better in all FPS situation as you seem to think, I believe the two are just different, and which you prefer will simply come down to preference and particular situation.
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