If I had to pick between the two of them, definitely Slipknot. Never really been a big fan of Metallica aside from a couple songs of theirs :P

15 years later, I still vote for Metallica; seeing them live in concert was a great experience, and I hope to get the opportunity again.

How is this even a question?

Metallica. Hands down. 

I honestly couldnt choose but maybe metallica is just a bit more my favorite.. maybe.. 

ive been a fan of Metallica for pretty much my whole life, seen them live a few times.. 
actually they are the reason i started playing guitar all those years ago!

slipknot has a few songs i like but most are screemo music. and mostly about killing yourself. never a fan of that

These cannot be a serious discussion. It's like asking "The Beatles or The Village People?".  Metallica is in a class by themselves. I grew up in the 80's when they became popular, and let me tell you, a band like them becoming popular was no easy feat. Even tame bands like Poison were shunned by the mainstream. Metallica had to be that damn good to get respect and climb to the top. 

Slipknot is just another band. Not good, not bad, they are just there.

I saw both live at a festival - one headlined one night, the other the next. As far as live performances, they are honestly both incredible. I can't put one above the other.

Corey Taylor is so impressive vocally and arguably a more talented singer than Hetfield, but Metallica are much more popular worldwide. But Slipknot are also in a more niche and less popular genre overall and still managed to make themselves incredibly popular considering.

Honestly I can't choose between these two. Metallica and Slipknot both have some fantastic songs, and have incredibly talented people in their bands. Metallica happens to be more what I would consider "general public friendly" with their style of music than Slipknot which is why there's understandably more people voting for them.

I really couldn't choose between these two

I dunno’ both have their appeal. Apples to oranges.