Unless you are planning on getting into long range shooting, there is negligible difference between the 16, 18 and 20 inch barrels. You will see greater bullet speed from the longer barrels, which equates into greater terminal ballistics, but until you get out beyond 250 yards, there's not really any noticeable difference.
A 4-inch shorter barrel is easier to carry slung as it is lighter and less to catch on tree limbs and such.
And as to who makes good ARs,,,pretty much everyone except Olympic Arms....all the parts they use are made to milspec dimensions and materials. The higher dollar ones are machined to tighter tolerances and/or are made to be lighter and would be considered "match grade" rifles. You can get a great shooter from a DPMS, Bushmaster, Smith & Wesson M&P Series, Stag Arms, Rock River...the list goes on.