I agree. a trackball will be much easier to add now. if you want to add one later, you would have to take your panel competely apart to mount it. a gun can be added much easier later.
i enjoy gun games alot, but i have yet to add a gun to my mame setup. ( i do however have a playstation 2 in the cabinet for gun games)...but i cannot imagine not being able to play centipede and millipede on my cabinet.