First off, the recent success of Black Ice is due in no small part to ROCK BAND (or haven't you heard of it?). Otherwise, AC/DC has gone the way of the Rolling Geezers and any number of washed-up bands - they haven't released anything worth listening to in 25 years and the only reason to go see them is to hear them play their classics from the prehistoric era. They'll erect a pile of scaffolding and purchase the same off-the-shelf stuff every other metal band does, their fans will show up drunk, high or both and everyone will have a good time.
There is simply no comparison between U2's music and AC/DC's. AC/DC's music is depthless, mindless and in-your-face.. which is exactly the only thing that band could ever produce. U2's music is intelligent, multi-layered and invites and challenges the listener to actually THINK while listening to it... Not just slamming a Milwaukee's Best Ice in your trailer while you crank out Back in Black for the 1000th time.
As far as album sales figures go, they are one of the worst ways to measure musical relevance or compare bands. Just ask fans of Radiohead. Hell, today, you don't even need to buy a physical copy of music at all to enjoy it, you can just download it for free and it will never be counted as an "album sale". Besides, there is little doubt that ROCK BAND has more to do with Black Ice's success than any other factor.
And one more thing about "album sales". I first bought Fleetwood Mac Rumors in the late 70's on vinyl. Why? Because I was a teenager and that's what our family had - a "record player". Then in the 80's I got my first in-dash cassette deck and I bought my second copy of Rumors on cassette so I could play it in my car stereo. My third copy of Rumors I received as a gift in the 90's. It was a purchased CD. That's 3 copies of the same album, THREE (
)! My point? AC/DC's two best albums were Back in Black and Highway to Hell. They both came out when most people were still buying vinyl albums, which most people then replaced with a cassette copy, which then were likely replaced with a CD copy.
In contrast, U2's two best albums were Achtung Baby and Joshua Tree. I purchased Joshua Tree shortly after it was released on CD.. and guess what, I still have that very same CD today. It sounds just as good today as it did 20 years ago. Same thing with Achtung Baby. I have no reason to replace them and maybe never will. If I want to put those songs on my Ipod I can just rip them off the cd's.
So let's do the math. That's ONE album sale per fan for the two best U2 albums to a very probable THREE album sales per fan for the best two AC/DC albums. Think about that the next time you start spouting off about all-time album sales for any band that's been around longer than the CD age, including U2. Those numbers can be very misleading unless taken into the correct context.