I'll say this....Metellica has been around for over fifteen years now and their music has been evolving since. First couple of albums - Kill'em all and Ride the lightning - are their real heavy metal albums. The next two - Master of Puppets and In Justice for All - are still hard but with better musical quality (they actually took lessons to learn to play and compose!) Next, the BLack ALbum, this is the album in which they really tone it down to explore a softer side. Not the driving-edge metal we had seen in the previous albums.
A couple of years later they put out an album called LOAD. This one was a more relaxed version of the Black album! Later, in conjunction with San Francisco philharmonic orchestra, they produce the S & M album. A little taste of rock with classical sounds. After this they produced, RELOAD...songs that got left off of Load.
So, to sum it up. If you want hard edged metellica music, go to their first four albums. If you want less edge, start at the black album (enter sandman, sad but true) and work your way forward.