I'm sure this has been asked, or something similar. But I searched before I asked and couldn't find anything. I'm not sure which songs to learn to help my playing. I don't want any suggestions like Metallica, Black Sabbath or old school, not heavy, metal/rock bands. I like bands such as After the Burial, Dance Club Massacre, Darkest Hour, Alexisonfire, AGATG, Emmure, 3 Inches of Blood, HORSE the Band, iwrestledabearonce, etc. I can only play a few songs and I'm just not sure what I should be learning technique-wise.

Play hard stuff... that's a good way to get better. Just practice it slow though.
You should learn palm muting, pedal riffs, arpeggios (sweep picking, string skipping), minor/harmonic minor/diminished scales.
Would it be more or less helpful, or just as helpful, to learn a really difficult song slowly than to learn a simple song? A song I'd want to learn is Berzerker by After the Burial but I don't think I could now.
Sounds pretty hard for a beginner. What I'd say is pick easier parts out of the songs like that and practice those for a while. Easier metal songs might help you more if you're a total beginner because songs like these have unusual time signatures and it's hard to stay on beat; also ATB has a weird tuning and 7 string guitars.
ATB uses 8 strings, but hafdahl is a genius. chill guy too.
Lamb of God has some good songs to practice, try Laid to Rest, or redneck.
Maybe Knee Deep by JFAC would be good, if you can play it.
good luck!
How about a high five?

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Defeatist by Hatebreed should be a good starting point
Andy Fox
Hard rock guitarist
I play a Jackson DK-2 and an Ibanez RG through a Peavey 6505+ stack
Thanks all. :] I think I'll try some of your suggestions, not sure about Laid to Rest. May be too hard. :P
Six - All that Remains has a nice variety.
Selkies: The Endless Obession - Between the Buried and me same again, nice variety of riffs and a great solo to finish off with.