I have a 2-player cab that rarely gets used as such. The 2nd player's joystick gets used for Crazy Climber and Battlezone, but not much else. I'll also note that 4-player CPs are frequently BIG... sometimes really too big for my tastes.
Of course you need to take into account what games you play and (as markvp noted) what your family/social circles are like, but I'd suggest you utilize the CP real-estate for a nice 2-player setup and include some other things you might want like a trackball, spinner, tron-stick, a dedicated 4-way, a sinistar 49-way or whatever. I know I get much more use out of my trackball and spinner than I would out of 3rd and 4th player controls.