There is definitely some tolerable Slipknot and even some songs by them I kind of enjoy. Can't say the same for Hollywood Undead though. Have yet to hear anything more than 30 seconds of any of their songs.
“But men, they say a lot of foolish things. In the end, the only words I can find to believe in are mine." - Joe
They both try to be nu-metal but as far as the songs they write, i'm not sure it's a fair comparison. I went with Slipknot because I listened to them for years in highschool. HU is good but I consider them more of a pregame band. Slipknot is just for when I'm pissed off. Plus I respect the hell out of Corey Taylor