There are 2 different styles, DPMS/SR25 and Armalite. Stick with the DPMS pattern as the parts are more prevalent and Pmags rule. Both of mine were way over gassed from the factory, JP adjustable blocks helped there. Factory DPMS barrels work well, I'm sure JP's barrels are fantastic as are the other quality barrel makers.
PSA has started to sell their own 308 parts as well. With the sale a few days ago you could have a 16" 308 for 600 bucks.