Personally I would go with a RRA AR-15. (Rock River Arms)
My father is Ex-Military and is a contractor for Academi (the former Blackwater security group) as a helicopter pilot and site manager. They issued him and the other contract pilots RRA M4's. He loves it and I have gotten really good feedback from others who own RRA AR-15 models.
Once I get extra cash put aside (in the process), I will be purchasing a RRA AR-15, Tactical CAR-A4 model.
One of my local FFL dealers can get the Tactical CAR-A4 model for $899.
So check them out...its worth it IMHO.