the civilian one will not have a high rate of fire
Follow up shots are faster than comparable "civilian rifles." Perhaps not sub-gun fast, but 4 controlled rounds a second sounds pretty dang quick.
wont have the armor piercing capabilities
Why would you want or need them? I agree, I wouldn't take on cops or the Natl. Guard with a PS90, either, but I also wouldn't do so, either
and with a 16" barrel hanging off the end wont be as compact either
Have you held one? They're still pretty dang small, just not concealable (which the real deals certainly are). Also more compact, again, than comparable "civilian rifles" like AR variants, other pistol carbines, or shotguns (especially)
might as well just go with a handgun
That is certainly an option, and many people do just that. I'm not an operator or even the least bit experience at this sort of thing, but I imagine that a two-handed weapon with a butt stock is considerably more stable, controllable, retainable, and more easily manipulated under duress than a pistol.
The PS90 is still very much in the spirit of the non-neutered version; light, compact, high capacity, effective enough to save your butt. I believe the original is select fire, so FNH/their customers must have seen
some utility in a semi-auto configuration. The auto fire mode is really there to ensure that close-in armored targets are hammered, since the cartridge, while fast enough to shear Kevlar (it doesn't
really penetrate) is borderline for that role and may not have enough energy once through to do the job decisively.
It's plenty decisive on unarmored targets, so special ammo less critical in the homestead role, as is the selective fire mode. I suppose you could argue it's a "drastically different" gun, but that's not to say it isn't very well suited to a much more practical role. I do agree that the long barrel sours many folks on the gun, since it is something of a "brand of inferiority" to mere civilians (it pissed me off and made me dislike the carbine, too, until I realized that it didn't really impede anything, and helped the cartridge deliver more force on target). At least I'd be less likely to shoot my thumb off with the anteater barrel
I'd be all over a SIG MPX in 5.7, since the locked breech would be a reloader's dream (I guess they're doing it in 357SIG, which is kinda similar), but it's more of a pistol AR than a PDW (plus I think the ATF's gonna go back on their word on a few things regarding that design before all's said and done)
TCB