I built my own AR15 using a DPMS lower, lower receiver parts, a DPMS A3upper with their charging handle and forward assist. My research led me to believe that Bushmaster barrels were the way to go so I purchased their barrel assembly, carrier, bolt assembly and upper handle. The main reason I selected the DPMS was its what my local gunshop carried. Our local law enforcement uses DPMS as well. I considered Rock River and Stag and they would have worked just as well in my opinion. Rock River was a little more expensive and Stag...well...its just me, but that logo on the side turned me off. There products are fine.
If you purchase any of the above mentioned "pure breeds" you will be very satisfied I'm sure. Bushmaster included. Mine is a hybrid...DPMS/Bushmaster. I really enjoyed building my own and would encourage those who have ever considered doing so to go for it. There are some great resourses here as well as AR15.com. You will definately have a thorough understanding of how your weapon operates as well as be able to perform your own maintenance and upkeep.
Some will say you can come out cheaper by building your own, and I would agree to an extent. You can definately make personal choices about various components and purchase them at your own pace (mine was 4 months). I however puchased the correct tool for the job (I'm a shop teacher) and as such have spent just as much as if I had gone out and purchased one. By the same token, I now have the the proper tools to perform more builds for friends and family as well as keep mine maintained.
More information than you asked for but I've been where you are and thought it would help. Others did the same for me.