I'm a big fan of both bands, but I chose Metallica. I'm surprised so many people are going with GNR though.
GNR's lead singer is a disgrace. He would let the audience wait forever before he goes on if he could. GNR, in general, just is very bad to their fans. The impression I get is they think their fans owe them something...like two hours of their time before they bother taking the stage (way past their scheduled time). I would never put up with that. While I like some early GNR music, I would never go to their show...just because you are not guaranteed they will hit the stage anywhere close to when they were supposed to. I guess it depends on how Axl feels at the time. Metallica even commented harshly about Axl in their documentary...when I think James got burned and Axl wouldn't go on stage when Metallica had to stop. Metallica blows GNR away anyway.