Funny but I was just asked this the other day...
regarding the HK-91/G-3 system over the M-14.
I believe the HK-91 system per se can retain its accuracy longer than the M-14. First, the M-14 is a prima donna that is very high maintenance. It requires constant attention to retain its accuarcy.
The HK-91 is actually simpler. Its weakness is the "flex" of the stamped steel receiver. This is rectified by additional metal reinforcing bars on the side of the receiver where a collaspable stock would fit on a HK-91A3. Going back to the system, because it's delayed gas blowback, it has fewer moving parts that the M-14 (no bolt op rod, op rod spring guide, piston (especially if the unitized gas cylinder assembly gets a bit loose) that can result in indifferent harmonics.
Now, if I had to drop the bucks, I'd go with an SR-25. I want that Remington 5R barrel. The AR-10T would be my second choice.