I also have both and I would give the nod to AFM. It just never gets old for me. I have had it for about 4 years and I still have games where my adrenaline is pumping, my palms are sweating and I am getting the shakes. It is by far the most played game in my collection.
MB is also awesome, but I haven't had it long enough to speak on it's staying power. I don't think it's as easy as some let on. I think those people don't fully understand the rules or have the machine setup too easy. I have extra ball limited to one. Lots of people seem to think Monster Bash is the end game, but it goes much further than that.
-Brian