I just finished putting an AR together from parts and I bought a complete Olympic Arms 20" SUM upper. I went with 20" for what I felt was max. velocity, less muzzle blast and weight.
The 20" is a bit on the heavy side but then again I pack a Rem. 700 Stainless Fluted 300 Win 26" deer hunting so maybe I just like heavy guns. They sure seem to hold steadier for me when the opportunity arises for a shot, whether walking or stand hunting.
I haven't had a chance to shoot it at longer ranges but wtih a Burris 4.5x14x42 and Win. White box 45 gr. HP it is shooting just under .5" at 140 yards which is the longest safe shot I can get from my picnic table in my backyard.
I have a bunch of cheapie, reloaded stuff, built with gunshow bullets that the best I can get out of it is .8" at the same range. That is still within MOC, minute of coyote.
I bought a stripped lower at a local gun show and then purchased the other parts off the internet. I think the best money I spent was in purchasing a Chip McCormick Match single stage trigger.
Buy what you want, the way you want it, the first time, it sure won't be the last.