If AFM and MM were the only pinball machines ever made then they are radically different games. But considering how much variety there is across the thousands of different designs that have been made, they are almost the same game.
This has nothing to do with the fact that I think MM is one of the best games ever made (I have an original).
In a small collection like yours, I would therefore advise you to get almost anything other than (almost) the same game you already have. BM66 would do just fine. Also not a good time to get MMr as the large screen update is not out yet.