I myself am researching my first AR-15 build. From what I've seen/read/heard, both are excellent quality. But, based on my own research, I'm going with RRA. It only boils down to cost. I guess Bushmaster has a more expensive logo? Or a better industry name recognition?
For my first AR-15, I'm going with all RRA parts, but I'm not ordering it from RRA. Eagle Firearms has RRA parts at a better price. For example, the rifle I want to build/buy is:
upper: RRA Standard A4, Chrome Moly, 1/9 Twist, No-Ban Model, 20" Flattop Upper Assembly (.223 Wylde chamber)
lower: RRA Complete M4 Lower 1/2 with 6 Position Stock, with National Match 2 Stage Trigger Group
From Eagle, these parts would cost a total of $718. Direct from RRA, they're $870. Hmmmm...... :?
For anyone interested in Eagle Firearms, here's their RRA products page:
http://www.eaglefirearms.net/Rock River Arms.htm
I'm going with RRA for my first AR-15. I doubt I'll be disappointed.