I think it really depends on your HD strategy. I for one would not go around clearing rooms with any gun. I would have a safe room that I would hole up in while I waited to police to arrive. So if I'm just keeping the gun pointed at the one door an intruder could enter the room through, I don't think it matters if the barrel is 18" or 30"
If I planned to take a more offensive approach, I'd be more concerned about barrel length.