"I'm thinking of long term keeper on the value of replay and nothing else."

That statement above makes me say Alien. Alien is going to have 15+ objective based modes in it that work the entire playfield. Both AFM and MM are awesome games but the main modes on both are all about hitting the same shot 5x, then 10x, then 15x, etc. AFM and MM gameplay is fun but I'm not sure if the rulesets for both would keep me interested after a year. I think that MM has the deeper and more engaging ruleset compared to AFM.

From the videos I've seen of Alien I can tell it's a game where the modes are unique and ones that work the entire playfield in interesting ways. Also, Alien has 4 flippers all with unique shots, color GI + color inserts, 3 ramps, Alien toy that eats pinballs, mutiple LCD screens, etc. Overall Alien seems like a very packed game.

Best way to make a final decision would be to get some gameplay on all 3, will need to wait for Alien since they are just starting to be shipped, and go with the one you like best.