#1
Single note riffs or chord riffs....

By single note riffs I mean what you'd think of as a riff...Black Dog, Iron Man, Smoke on the Water(Even though these last two are more than single notes...)

By chord riffs I mean like Bigger Boys and Stolen Sweethearts :
http://youtube.com/watch?v=SCgyR50l6t0

Or Walking on a Spiral:http://youtube.com/watch?v=hwo_ZH8gnHk

Which do you like more, or does it not matter?
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#2
I try incorporate both styles when I write so I would have to say it doesnt really matter
#3
Chord riffs for sure, that doesn't mean i dont write both, but u can put a lot more atmosphere and dimension with a chord riff incorporating 7th chords and modified 7th, sus chords u know put lots of mix match stuff and see what comes out
#4
I never classified riffs like that... I guess most metal licks would be single notes picked, but it all come down to the sequences played, the harmonies between the instruments and the way it's all played and mixed... so... lot's of variables. For DM I usually do single note sequences all the way through, except on solos and melodic sections... for HC I do picked stuff for everything but the choruses, which are usually rhythmic passages of extended chords.