A couple of things to add:
I own a DPMS and an Armalite AR-10 target tactical.
I've had zero reliability issues with either rifle. The most common cause for reliability issues with the AR-10 rifle is the magazine.
With the DPMS the polymer mags that originally shipped with the rifle caused a number of problems. This was fixed with the current metal mags which are excellent (compatible with original AR-10, Remington R-25 and Knights SR-25).
The Armalite rifle ships with a Generation 3 mag which is a modified M14 magazine. I've seen first generation mags and they are very different than the generation 3 ones. Again reliability issues are most likely to be magazine related.
Which brings us to the Rock River Arms Rifle. This is the failed Bushmaster AR-10 rifle they couldn't get to work. RRA bought it and took a long time to get it to market. While I do trust RRA to make a good product (love their AR triggers), I suspect that the main issue with the rifle was getting a FAL mag to work with the AR-10 system. Armalite had an excellent article on why they chose the M14 mags and not the Fal ones. It again went to reliability issues. The main reasons for Armalite's choice was the assault rifle ban, availability and reliability.
Of the three rifles I like the current DPMS metal mag system the best. There's no comparison.
I wouldn't count on getting one of those Armalite rifles for under 2K. The inexpensive uppers were very well used ex military uppers. There are a number out there that were only lightly used and not military issue, however they were very hard to find and cost a lot more. Then there is the issue of finding an AR-10 lower (The AR-10 rifles of various brands aren't compatible with each other. Ie you need a lower and upper that will mate. A DPMS lower won't work with an Armalite upper). You're generally looking at 3K for a new Armalite, or a very lightly used tactical target model (If you can find one).
On a side note, you'll need to upgrade the triggers for both. The Armalite "match" trigger is much better than the stock DPMS (Which really sucks). However it's not that great. The RRA two stage match works well. There are better but the price goes up.
DPMS LR-308 on the left. Armalite AR-10 Target Tactical on the Right.