I would take the Stag first, over the RRA. Let me explain the difference, both are machined by CMT, but RRA has a slightly different spec from Stag, Stag makes a small modification to make their lowers easier to work with, RRA lower receivers are milled slightly undersize in the rear lug area, I'm guessing that they spec that for hand fitting at their factory, Stag receivers are relieved in that area, simply put, they go together easier.
Some might think tighter is better, but it really doesn't make any difference at all, the fit between the upper and lower on an AR has no effect on accuracy at all!
For the person assembling an AR, the Stag is easier to work with, quite a few first time AR builders complain their new builds using RRA receivers don't fit, and they don't want to go grinding on their brand new receivers!
I've built AR's on both Stag Arms and RRA lower receivers, I'll take the Stag any day over the RRA.