AFM & MM are at the same time completely different games in every way as well as almost exactly the same game depending on how you look at it.

If all you ever saw and experienced were these two games, you would not see any similarity between them. Like if the only fruits that ever existed were oranges and tangerines you would love them both in subtle and complex ways and think they were completely different.

On the other hand, if an advanced alien race systematically studied every pin ever made by us humans, then, in the aspect of playfield layout and flow, AFM and MM are very similar to each other. In the aspect of playfield layout and flow, they are similar like how you think of oranges and tangerines given that you have experienced a large variety of fruits (sweet citrus fruits). There is a reason for this, MM is a modified version of AFM in the way that they started with the layout and code of AFM and added/removed things and made a different game. "What if we had a castle and had a two stage drawbridge." "What if we added some trolls that popped up and blocked shots". Etc.

IMO, both are fantastic games, each a pinnacle of the pinball artform for a large number of reasons. I think it's nuts to say one is the best and the other sucks - like saying out of the top steak restaurants in the world, one totally sucks over the other.

Now when you hear someone saying "don't get both if you're new to the hobby" what they are saying is that AFM and MM are similar enough that you should probably try a different kind of game (say a stop-and-go like TZ) so that you experience a wider range of pinball layouts. They are right to say this. But those who say they are both awesome are also right.
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(looks like I must be getting hungry given all the references to food