They’re both great, and really very similar. Overall I prefer MM but I wouldn’t turn down an AFM... however I don’t think you need to own both.

As for why MM? Theme, art and sound are subjective so I’ll leave them out of this...

Multiball Madness. It’s super cool to stack all the various multiballs and them pop them all off at once... it’s basically an evolution of Total Annihilation.

The standard Multiball mode where you’re killing the knights is a little more interesting than just shoot shoot shoot super jackpot...

The ability to hunt castles in multiball... AFM will let you kill an open saucer during MB but not open another. MM you can actually keep qualifying castles. This gives you more multiball strategy in MM.

Trolls are more fun and cooler than the bouncing martians and targets. They also get in the way, which is more of a challenge.

The Battle for the Kingdom endgame on MM is also more satisfying, it’s kind of an evolutionary step from AFMs.

I had them both once. Started out preferring AFM, ended up preferring MM... it’s just AFM perfected. You can see the design lineage and how Lyman and Eddy set out to improve certain things.