Velocity of the round changes when you shorten a barrel, accuracy does not. So the .223 will not shoot "as far" so to say with a shorter barrel, but is still more than adequate for 100 yards with shorter barrels.
Muzzle blast and noise increases dramatically with shorter barrels, it's already loud enough on a 20".
I personally prefer the shorter barrels just cause they are much more easy to carry around, manuever and sling to your body than longer ones.
I have an 11.5" and it's been just awesome the past 2 years.