First of all, I think it was well covered about how the shot gun is tactically better in certain situations vs a sub machine gun. Also, in other situations the Sub Machine gun is better. That's probably why Alot of SWAT teams carry them both.
As far as the bullets fragmenting on impact. Havent you all seen the Myth Busters episode where they see how far into watter a bullet will still be deadly?
They find that the higher speed rounds fragment basically on impact, making them lethal only about a foot into the watter, they even tried shooting a .50 cal sniper rifle with the same effect. Where the 9mm they shoot had the greatest lethal travel under watter, something like 4 or 5 feet. There for, also like said above, a long distance shot with a high speed bullet will slow the bullet down, allowing for greater penetration... where a close range shot, in which case the bullet is going much faster, it will fragment sooner upon penetration. makes sense to me
I'd also go with what was said above, that the semi auto shotgun seams to be the best of both worlds. It's almost like cheating
But, it will never give you that precise shot of an SMG... there for SWAT caries them both