They do different jobs. It's a matter of swings and roundabouts.
Ballistically, the low-calibre rounds of a PDW (as currently conceived) will make smaller holes than the mid-calibre rounds of an SMG, but terminal ballistics are a very poor measure of the lethality of a weapon, or the battle rifle wouldn't have evolved much past the arquebus stage (very large lumps of lead moving fairly fast).
The fact that elite hostage rescue teams haven't adopted the H&K MP7 or the FN P90 wholesale would suggest that the SMG is still more lethal at that sort of engagement range and against the target types they'll typically be facing.
Ballistically, the low-calibre rounds of a PDW (as currently conceived) will make smaller holes than the mid-calibre rounds of an SMG, but terminal ballistics are a very poor measure of the lethality of a weapon, or the battle rifle wouldn't have evolved much past the arquebus stage (very large lumps of lead moving fairly fast).
The fact that elite hostage rescue teams haven't adopted the H&K MP7 or the FN P90 wholesale would suggest that the SMG is still more lethal at that sort of engagement range and against the target types they'll typically be facing.
