Re: Armalite lower/DPMS upper?
No go.
As there is no universal 308-cal AR blueprint, everyone does their own thing. Armalite, DPMS, and Rock River all make their own thing - and none works with the other.
I prefer Armalites, opinions vary. I have noted more quality issues with DPMS than Armalite (going back quite a few years, admittedly.) Rock River is fairly new to the 308 party, and their stuff isn't as mainstream yet. But I hear good things.