If you are choosing between MET and AC/DC, pricewise I think your choice is a HUO AC/DC Premium, or a NIB MET Pro. I have spent some time with both flavors of both machines (Pro and Premium), and think that AC/DC Pro isn't in the same league as the Premium, and MET Premium isn't that much different than the Pro.
I thought for sure I would like MET better than AC/DC, as I really like Ironman, and MET feels like a little deeper version of Ironman to me playwise. I really like the artwork and really like the band/music, so MET was the no brainer choice for me. I cannot stand the band AC/DC and I am not an over the top Steve Ritchie fan, generally. The artwork seemed fairly uninspired to me, and the plastic dancing band seemed really cheesy.
After I actually played both machines, I couldn't stop playing AC/DC. Whenever there is an AC/DC at someone's house, I gravitate towards it, and have a hard time not playing it. I don't feel the same way about Metallica as it sits right now. I really like the game, but could take it or leave it. I have been having a very difficult time NOT pulling trigger on an AC/DC, the game is just flat out fun, the shots are great, the lighting is second to none, and the rules and the way you go about socring/playing the game are fantastic.