If we are talking old Metallica then yeah Metallica.Anything after there black album sucks,that was basically there sellout album.Slipknot is pretty badarse I've seen them 3 times.How many of you even know that Dave Mustaine(Megadeth)was the lead guitarist and co-wrote a couple of songs on Kill Em All and Ride the Lighting after only playing two years with them got kicked out of the band for beating the crap out of Jame Hetfield.
Tough choice, both of them only have a few good songs. Slipknot has a few really good songs, while Metallica has more lower quality songs (in my opinion). I listen to Slipknot more often though, so I'll have to go with them.