Demon sin - better for farming - triple spark kills. Sage sin - Better chi management - higher rate of survival with triple spark + sage tidal.
I think sage is just as useful for farming if you want to do instances like TT2-3/3-3 where the bosses hit hard and can throw out lots of debuffs.
Yeah, I hear that "_skai" guy is pretty good xD
Go on youtube, look up ZSW Pw. Or Flash's vids here, study how they play (or Skai's vids on youtube, they're all decent sins) learn a thing or two, have fun.