I have owned both a beautiful restored AFM original and a Metallica Premium. I miss both but cant afford to keep every pin that comes through my gameroom with prices what they are these days!

Anyhow, the answer to this question is that unless you are super in love with the nostalgia and theme associated with AFM, Metallica is a better game in pretty much every way. The code/rules are fantastic, the artwork is among the best of any Stern, the layout is fun and feels great (looping the metal ramps is super smooth!) The depth of gameplay makes it approachable but yet a very nice challenge that never gets old.

IF I could have one of those two machines back it would be the Metallica for sure.

Considering you have an MMR as well I think this is a no-brainer. You already have the awesome old-school fan layout department covered with your MMR, you will not be sorry replacing AFM with a Metallica.