Let me expand on Rich314's answer.

The 16" barrel is a great carbine length barrel best for a carry rifle - something that might see "social" use - you know you and someone else exchanging fire. It will point quicker and get hung-up on less off-range. That extra four inches of barrel indoors is tough make corners with too.

The 20" barrel will give a given piece of ammo more velocity. Higher velocity means more energy to transmit farther down range – important for longer shots say - 250 yards and beyond as the .223 has a magic 2700 ft/sec fragmentation velocity. [I]Personally[/i] if I wanted to shoot anything but paper beyond 250 to 300 yards I’d step up to a .22-250 bolt rifle for the varmints and a .308 for something larger.

Both barrels can be as accurate as the other. Where the extra 4 inches comes in and its extra velocity comes in handy is punching paper at distance. Less time in the air means less time for Mother Nature’s forces of gravity and wind to act on said flying bullet.

What's barrel whip? [>:/]

My 26" long bolt action bench rest rifle must not have much of it because it shoots sub-half minute of angle shots.

If you're talking harmonics you're in a totally different science than pure length - weight, shape, hand guards, crown and a dozen other factors need to be taken in. The only practical way to manage it is with an adjustable stabilizer like the Browning one.

I’ve not meant to flame but let’s not confuse what seems to be an entry-level shooter. If 19Delta is building an unlimited class $3000 space rifle then we’ll talk harmonics, free floating, titanium and other weird science. [;)]

M193 and M855 won't fragment unless they strike their target doing 2700 fps or better.  When they fragment, they make very large, nasty wounds.  When they don't fragment, they merely make a deep .22 caliber hole.

Range where velocity drops to 2700 fps (these are approximate, and can vary slightly):

M193
20" = 180-200 yards
16" = 140-150 yards
14.5" = 90-100 yards

M855
20" = 140-150 yards
16" = 90-100 yards
14.5" = 40-50 yards

Rangers in Somalia went in with either 11.5" or 14.5" "CAR-15s" and M855 ammo.  While they anticipated close-quarters work, they ended up in a prolonged gun battle on the streets of Mogidishu.  Several of them reported very poor performance of their weapons system.  Though there are a number of contributing factors (thin bodied attackers who were likely somewhat drugged up on khat, a local plant), the combination of short-barreled carbines and M855 ammo are likely the primary consideration.  Lacking enough velocity to fragment at ranges beyond 50 yards, the M855 wasn't wounding the Somalis enough to put them out of the fight.  The Rangers who came in via the truck convoy, who were armed with 20" M16s and M855, did not report these problems.

This is only a single incident, but it is instructive.  Military .223 FMJ ammo relies mainly on velocity to be effective, and therefore shorter barrels reduce effective range.  Effective range being defined (here, by me) as being the range in which the weapon system will reliably cause wounds severe enough to take an enemy out of the fight.

This is something you may with to consider when selecting a barrel.

-Troy