I bought a Standard back in 2004-6 (can't remember exactly when). New SA receiver (cast), 22" new (not chrome lined) barrel, NM front sight blade, muzzle brake (no flash suppressor in CA) and a USGI fiberglass stock painted with a black crinkle textured paint.
I love my SA M1A. It's more accurate than I am. Having said that:
Right out of the box, the rear sight bottomed out and I was still shooting 5 feet high at 100 and 200 yards.
Called SA and they were helping me diagnose the issue. They suggest I go out and shoot it again and report back. 2nd time out, the elevation knob broke off (looks like a cast part from un-even-ness of the break). Still shot high before the rear sight broke.
Called SA and they immediately sent me a new rear sight assembly. They were very apologetic and genuinely interested in resolving my issue.
In the meantime, I pull the rear sight off my CMP Greek M1 Garand and shot with it (still on the M1A to this day). Didn't even need to adjust the sight. Left the zero settings from the Garand... I'm right on target. Nice. Very accurate, easy to aim, low recoil, easy to carry (for a shrimp like me @ 5'7" tall). The new SA rear sight came in the mail a few days later and I tossed it in the safe (never to be used).
Then the obsession started. I found m14forum and immediately started replacing parts with new surplus USGI parts: TRW bolt, op rod (GI SA), H&R trigger group, and a genuine M14 walnut stock from Fred's. The only aftermarket (non USGI) part was the Sadlak National Match spring guide. I think I dropped at least $500-$600 for these parts (maybe more, since this is the amount I admitted to my wife). USGI parts were hard to get, even 5-6 years ago.
I also bought cases of South African surplus ammo (remember those days?) and shot almost every weekend for about a year. It's the most accurate rifle I own. It never disappoints me and there's always someone asking about it at the range (say... is that a Fulton M1A?).
Back in the day, SA M1A's would occasionally come out of the box with one or more USGI parts on it, but those days are long gone. Now that USGI parts are even harder to get at reasonable prices (if not impossible).
If I had to do it all over again, I'd spend the extra money on a Fulton, LRB, etc, so I wouldn't have to deal with things like a rear sight that fell apart, etc. Lifetime warranty gets old when problems actually occur. Plus, as I recall, the LRB and Fulton receivers are forged and chrome lined barrels are available.
I feel like I can't mention it enough, but I absolutely love my SA M1A in it's current configuration.
It's just getting more and more expensive to shoot 308 and I don't reload.
As far as the AR platform vs the M1A platform: My 20" barreled AR (223) is also very accurate, but I think my M1A has the edge. But to be fair, I only shoot cheap (really cheap) stuff through my AR.
I've never competed, nor have I ever bought expensive match ammo for any of my rifles. But from what I've been told by guys who re-load, cheap surplus ammo (or anything made by Wolf) is not what you want to use to gauge the accuracy of one's firearms.
Good luck and have fun choosing.