I was hoping the instructor from the first video was going to beat someone on the head and say now that's thinkin' with yer dipstick jimmy.
I have thought about this and in my unprofessional opinion a longer barrel would be better for fighting enemys who also have a bayonet or close combat weapon because you have more reach. A shorter barrel would be better for killing unarmed enemys because you would have more control and less of a chance for the enemy to grab your barrel. Of course with bull pups and short rifles you can't use skull crushing vertical butt strokes as well.