i have an ipac but i've been having some strange issues with some connections not working anymore (granted, i've had the thing for about 5 years and it's been in at least 4 different machines, so i don't hold it against ultimarc)
honestly, both suppliers are top notch and there is nothing wrong with either option
there really isn't a wrong answer here
the last thing i would personally do is hack a gamepad though
although not exactly hard....to me, it's more of a pain with software configuration
some things use joysticks/gamepads.....all should use keyboards (i'm assuming that you're not going to put a console in your arcade since you're inquiring about ipacs and key wizes)
if you want cheap, hack your own keyboard
if you want quality, go with the ipac or the key wiz
in case it matters, i'm going to be buying myself another ipac here in a month or so...so that gets my vote....but again, it really doesn't matter...they are both great