Which is better IYO, the Black Ice, or Death Magnetic?
They're both definitely great albums, but as Howie said, you have to measure them with a different scale.
Death Magnetic rocks your face and balls off. At worst, it's Metallica's most rocking album in over 20 years. At best, it might possibly be their most rocking to date. It sounds like Master Of Puppets + Rick Ruben's Midas Touch. Add in the benefit of modern recording technology and it's just a better sounding record in my opinion.
Black Ice sounds exactly like every other AC/DC album they've ever done. From the first record they've basically sounded exactly the same, with the exception of Bon being replaced at vocals. Bad ass riffs, backed by steady paced rock. This album could have just as easily come out in 1976 and High Voltage could come out today, and one wouldn't sound stylistically different from the other. In my opinion, this is refreshing since most bands get old and sober and significantly decline their rockage. If it ain't broke, don't fix it type of thing.