Megadeth are similair to Metallica, because David Mustaine, the current lead vocalist of Megadeth, was originally in Metallica. However i prefer Megadeth, they perform better live, and their solo's are much better. Also, i think they're better lyrically as well.
Although i am going to defend Slipknot at this moment in time. They may be Nu-Metal, but if you look at their newer stuff, they're drifting away from their Nu-Metal Roots and are becoming more mainstream, which sucks anyway, because i think they were better when they did Self-Titled/Iowa.
Going back to azuman's comment, I hate going into HMV, and finding that My Chemical Romance (i like to call them My Chemical Re***ds) is in the Metal Section. I just want to pick them up, and transfer it over to the Rock/Pop Section. It makes me sick seing them there, mainly because they aren't Metal, they're EMO, because their lyrics are so dreary and depressing, and so's his voice. Even though they say there not EMO then you look at their hair, their image, and their music, it all comes back as EMO.
And i listen to a lot of growly, deathcore/grindcore/death metal stuff nowadays, such as Lamb of God, Cradle Of Filth, Suicide Silence, Waking The Cadaver, Job for a Cowboy, Whitechapel, Deicide etc..
But i do like all my non-screaming stuff, like Avenged Sevenfold, Disturbed, (i have all their studio albums now, yay :D) Down, System of a Down, Evile, Iron Maiden, Megadeth, Black Sabbath, Ozzy Osbourne etc.
Sorry, its a bit of a long winded post guys, but i had a lot to get across to you. :)
EDIT; Paul Gray and Joey Jordinson = God's!!!! :D