The Armalite probably has a mid length gas system, I dont believe the RRA you mentioned has that. There's "slight" ups and downs to both guns, but I say slight because I believe the two guns are very comparable. The ups to each will be the down to the other. In my opinion:
The ups for the Armalite:
The Armalite will have a M4 style profile to the barrel meaning it will be thinner under the handguards. I think most people appreciate the weight savings this offers. The Armalite will have better M4 style feed ramps in the reciever than the RRA. The Armalite comes standard with a chrome lined barrel, thats an upgraded option on the RRA.
The ups for the RRA:
It will have a Houge grip which is nicer than the standard. It will have the RRA 2 stage trigger. Its not an awesome two stage, but it is nice and it is certainly nicer than a standard milspec style and weight trigger (like the Armalite will have). The Rock Rivers usually have a nicer fit, finish and furniture than the Armalites.
Between the two, I'd take the Armalite. One, because it has the standard chrome lined , lighter weight barrel. Two, because its available right now!
But you're getting a great gun either way you choose.