I have owned both personally...
The Armalite barrel is definitely a slimmer profile. It's not as thin as the government profile, but the RRA barrel is a full, non-tapered HBAR all the way to the gas block! It's unreasonably heavy in my opinion.
The RRA barrels do have a chrome lined option (sometimes), but I'd definitely go with chrome lined unless $$$ was an issue.
If you go with RRA, check with ADCO as they offer the profiling option for just a little more $$.
At the time I was building mine (about three years ago), RRA and Armalite were the ONLY games in town. I didn't keep the Armalite because it was a complete upper, and all I wanted was the barrel. I kept the Rock River, BUT I sent it to Kurt's Kustom to have it turned and fluted. It now weighs about the same as a medium profile barrel, but it cost me nearly $300 for the barrel and work. I also ended up buying one from ADCO with the profiled option (ADCO started offering them about TWO WEEKS after I sent mine to KKF) with the intention of selling my heavier fluted one, but nobody else was interested in it either :) Every time I mentioned having the two barrels, I'd get "the fluted one's nice, but how much for the ADCO barrel?"
PERSONALLY though, if I were just now building a mid-length, I'd look long and hard at the Sabre Defense mid-length barrels. They are already government profile, and will cost about the same as the other two barrels.