I have both a Bushy A3 with a 20" barrel and a Rock River Entry Tactical M4gery. Both are excellent weapons, shoot great and are very reliable.
I would say the Bushy is closer to mil-spec. The Rock River is more law enforcement spec. That being said, the Rock River was chosen by the DEA and FBI after grueling reliability tests.
All things being equal, I think you get more goodies for the buck with the Rock River. Mine came with a 2-stage trigger, a Hogue pistol grip, and Yankee Hill free float quad-rail handguards. I had to spend extra money for these items on my Bushy. However, if you want top of the line stuff, buy the Bushy, and then customize it. Free float the barrel, put in a 2-stage trigger, change the pistol grip, and on and on.
I don't think you can go wrong with either one. It's just a question of how much you want to spend.
BTW--don't forget the cost of a good scope. I have a Leupold MR/T 3-9x36 scope on my Bushy and an Trijicon ACOG TA-31 RCO M4 on my Rock River. Plan on spending about $1,000 for either of these.