From a vehicle, a shorter bbl makes a difference......
as one gets it from the rack, and deployed in what ever manner, from the veh or out on foot. I notice a distinct difference between 18 and 20 in such situations.
For HD, which would have more space than the confines of a vehicle, I'm not so sure. The longer bbls do allow for more mag tube space, which is one of my arguments against the ultra short entry guns.
Seems like I read somewhere that some name trainer advocates the use of RS sighted, hunting type slug bbls lengths, to avoid possible civil entanglements. Somebody else started that, not me. Seems like a case of the tail wagging the dog.
Since you asked, I'd go with the 20", and invest the money you save on night sites, weapon light, and/or an ammo caddy of some sort.