The PDW is a great little device... but I think the P-90 would have it beat due to it being smaller (when the PDW is shouldered), the more controllable RPM rate, and the supeirior ballistics of its cartridge.
I fired the PDW and frankly would prefer a "full sized" MP5. But we are talking SMGs here and have gone beyond the Machine Pistol topic... PDW being an albeit small SMG, a SMG none the less.
I know this sounds stupid - but perhaps a Glock 18 chambered for .380 would be ideal. Since the 9MM needs a great deal of practice to master. A .380 version would be much easier to control.
Imagine this - a fully auto Glock in .380 based on a Glock 19 sized frame. Gives you a lot more concealment options than a PDW, controlable, and you're seconds away from a nice long 30 round stick magazine when the fecal hits the fan. I'ld buy one. Because I can't hit very well with an '18...