Metallica hasn't rivaled Slayer in many many years, not even in the same class anymore - Hiring Rubin to produce an album might work for Run DMC and Def Leppard, but Metallica needs to get the heart back. I'm sure they have sold way more albums than Slayer, but in doing so, they alienated the fans that got them there in the first place. There are usually 2 Metallica camps - the early stuff, and then the new stuff. Slayer fans embrace the whole catalog generally, and they consistently produce metal albums that set the bar/benchmark for the genre. Add in the live show aspects and theres no contest IMHO - SLAYER rules