A 20" barrel offers a longer sight radius and higher velocity, both important when the target is 200 or more yards away. A 16" barrel is more portable and maneuverable, but it uses a shorter gas system and therefore a shorter sight radius. Many people prefer the shorter barrel for lower weight, but in doing so they sacrifice the advanatages of higher velocity and sight radius for convenience. But since most people shoot at 100yds, and the sights work just fine for going bang, they never know the difference.
Never try to educate someone who resists knowledge at all costs.
But what do I know?