I've always been partial to the 20" AR's. I like the sight radius and the length is more of a blessing than a detractor. I'm not out kicking in meth lab doors with a team so don't need a shorty. I've shot a 16" bbl AR before and didn't like the increased muzzle blast over the longer tube.
Besides, the 223 is sort of a pipsqueak round anyway, and can use all the velocity it can get, ergo longer barrel.