Why aboo, you make me blush...
I am a big fan of both bands. I have seen GNR live. Every time I plan to go see Metallica, something prevents it.
GNR rose very quickly. They literally took the music industry by the bawls and led it around for awhile there. The problem was, the band members didn't like each other. Lots of in-fighting. Another problem was they had less than zero respect for their fans.
Metallica was one of the more honest bands around. IMO, the day Cliff Burton died, they became a different band; they went from dead set against videos to MTV whores. They used to openly encourage fans to bring tape recorders and video cameras to the shows and bootleg them; now they close Napster down. Who cares about the haircuts a few years ago, hair don't play a note.
The big difference to me is simply this: I still like Metallica's music. Reload was awesome, Garage Inc. was fascinating, could care less about S&M. GNR's last few CDs were hideous.
I'll still take KISS, AC/DC, W.A.S.P., or Judas Priest over either of them for the hard stuff.