i use both, using a track ball is by far the best for moavability and close range machine gunning, while a mouse is better for long range rifles ( abit more steady when aiming).
I switch between the two when changing weapon.

Also as i am an animator/designer I have a huge wacom tablet for my work(pen stylus) which i have tried in fps games this gives pinpoint accuracy for sniper rifles.

If someone does decide to get a trackball, i suggest give it a chance and dont give up too soon as it can take some weeks to get skilled at it, but once there you wont look back.

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