I work at a small FFL and do occasional armory-level fixes for people. As I recall I've had two Ruger ARs come through the door for repairs and the quality control on both were pretty poor, one of them didn't even have all the retention springs in the receiver. The other hadn't had the buffer tube screwed in all the way and when I opened it up the buffer retaining pin shot out.
Now while this normally might be due to Bubba the gunsmith/tinkerer, I am certain that neither of these two guys were the type to open up the gun. It's hard to explain but it's easy to read when you talk to them, they'll gingerly clean their guns but they're under the impression there's something kind of mystical about working on them, not the type to swap out furniture on their own.
That's just my experience with two rifles, it shouldn't be taken as anything all-encompassing..
The second thing that worries me about Ruger is the use of UTG parts. I hate that they have sloppy QC with their tactical rifles, in my experience/exposure, because they make damn fine revolvers of which I own several.
My friend owns a Sig 716 and we go out and shoot together a fair bit, I've been behind it 6-7 times. Never had any issues with it, and he hasn't had any complaints. The only thing I dislike is how front-heavy it is, but being the owner of two LMT MWS rifles, it's nothing new, a Magpul UBR or PRS stock does wonders to "balance" them out, but it'll always be a heavy pig, relatively speaking.
I'm assuming by your choices you're looking to stick with a piston system? If not the entry level DPMS and new Armalite B models (accept SR-25 pattern magazines), are great for the money. I'll admit I was a little sour when my buddy shot a very close group to mine with his $900 DPMS he added a free float tube to, compared to my $2300 LMT, which is pretty much why I build my own now.