A 20' barrel will mean higher velocity, and a more accuracy at longer range. Shooting the AR at extended ranges, the 150 to 200 fps loss in velocity make make a big difference in bullet performance in terms of how the bullet acts when it hits it's target. In terms of accuracy, the shorter barrel is 'stiffer' and introduces less "whip" than the longer barrel, which affects accuracy. However, the shorter barrel can not generate the higher bullet velocities that a longer barrel can, which in turn affects bullet drop much more than the longer barrel. The 20'' will generate better velocity, which translates to better accuracy at long distance along with better terminal performance. Perhaps this why I have never seen a carbine used at a competition match.
NRA Life Member (2003)
I think that one of the notions common to the anti-gunner is the idea that being a victim is 'noble'; as if it is better to be noble in your suffering than disruptive in your own defense.