Anthrax are good, but I've never really found myself sitting around listening to their albums for a good few hours. I'll throw on an album, and I'll usually get through it, but that'll be it. With Slayer I can thrown on and album, and then throw on another and another.
Having said all of that, I've grown increasingly tired of Kerry King over the years, especially from around the God Hates Us All period. I just feel that he's become too much of a caricature.
Don't get me wrong, he still writes great songs, but I've always felt that out of the two, Hanneman has always been the stronger songwriter.