Admittedly I was exaggerating when I said that a PDW has the same pros as a traditional assault rifle like the M16, but I stay by support of the MP7 as the perfect anti-zombie weapon. It has a range of about 200m, which is what I'd call maximum range for zombie-hunting, because engaging at anything longer than that against a target that is effectively the size of a baseball is a waste of ammunition (unless you bring out the big-caliber guns and decide to literally tear the undead apart from outwards of a mile). And, quite frankly, if slow, shambling corpse-eaters were over 200 meters away, I wouldn't give a damn trying to kill them anyways: they aren't my problem.
And while the 4.6mm round IS primarily an armor-piercing round, it carries twice the ballistic velocity of traditional pistols (more than three times that of the Colt 1911), and it seems like zombies don't really care how BIG the hole in their head is, as long as it's there. Even so, I'm fairly certain that if I was up against the undead and managed to find me an MP7, I'd find some spoon-tip rounds to blast big empties.
The best reason I can come up with for the MP7 being my weapon of choice is its size: I'd really hate to think that I'd be cornered by slow zombies in the middle of Times Square, or in an open field: if I'm getting jumped by the undead, it will be indoors and in close spaces. I can pump an awful lot of 4.6 into a zombie when he bursts out of a closet, but if I was carrying an M16, it would just get in the way.