I voted MM. After having had the opportunity to play AFM and MM next to each other for a good amount of time, I had way much more fun with MM. I feel that AFM is very bland, and not much to do. For the money that the AFMr is going to cost, I'd purchase a SMVE which is very similar to AFM and have money leftover to also get a ST Premium.
As for Alien, it looks cool...but I'm not a fan of the theme. I'm also worried that as soon as people start getting them, the complaining will begin. Since the machine is different than what most pinheads are into, once the "cool factor" wears off, I see hate/flame threads in the future.
It's your money and ultimately your decision, but I'd recommend going to an arcade that has AFM and MM and put some serious play time into each, and see what game makes you happier. Best of luck with your decision.