The jpac is basically a jamma version ipac. You have about the same amount of button inputs, plus a video amplifier. The difference is that the jpac utilizes the jamma harness to connect the buttons to your PC, instead of the screw terminals of the ipac. So both encoders have the same function, just different connections.
Now if you want to stick with the ipac, because you don't have the money after you changed your mind, I believe Ultimarc sells a monitor amplifier for $15.