I'm partial to a 20", It's more of a classic AR length from a historical standpoint.
Eventhough people here will say a 16" has plenty of velocity to fragment properly, the additional velocity the 20" gives can only enhance the probability of fragmentation effect.
Also, I trust a barrel length closer to the original in terms of reliability of the gas system working when ammo is marginal.
The sight radius will be longer, making for easier long range shooting. I swapped in a Match front sight on mine, a $10 no tools upgrade, and the thinner blade really helps at 100 yards.
Also less muzzle noise and flash with a 20", though I think all Armalites come with a muzzle break, so be ready for some loud shooting.
In my opinion the only advantage to a 16" barrel is easier accessibility in a vehicle if you are going to be carrying it in a rack or behind a seat.
As far as CQB goes, as moderators here have said, soldiers used to clear houses with Garands, and I think 4" more of barrel adds far more advantages to it as an all around rifle. I can climb up the stairwell in my house well enough with my 20" and get a barrel around the corner pretty easy.