This is a tough choice. Both are excellent and quite possibly some of the best Stern has ever done, period.
Both game's feature unconventional rule sets but AC\DC is the more obtuse of the two. Despite the game being around for many years now I still see confusion about how the VIP passes work, how add-a-ball works during multiball, what the overarching goals are and so forth.
I'll echo that AC\DC is absolutely a more "score oriented" game than Metallica. It has less in the way of traditional modes and scoring strategies change depending on the song you are playing. There's also a huge focus on getting the Song Jackpot and trying to multiply its cash out for huge points. That's supremely satisfying, but completing a Crank it Up on Metallica is too.
Of course, there is a major (and awesome) wizard mode in AC\DC, but the journey to it is long, hard and not quite that apparent on first glance. Most will never get to experience it and so what they are left with is shooting lit shots for points. On Metallica you always feel like you are progressing towards something bigger (like a Crank it Up). On AC\DC that feeling's not really there unless you are specifically playing for Encore. You're not going to happen upon it by accident like you can a Crank it Up.
The best choice though? Have both machines.