I usually base my choice not on which one, but on what else the seller has that I need. I have built tons of Mame cabinets and IPac and Keywhiz are for all practical purposes interchangable. The feature set is mildly different, but the vast majority of users aren't going to be using any of the "features" at all. I certainly never have.
Right now I THINK both my mames have a keywhiz in them, but I have also used tons of Ipacs as well.