I'd go with a 300BLK. Full disclosure - I have one, so I'm a bit biased. That said, you get all the knockdown power you could want for home defense and, as a bonus, you get a boat load of parts commonality with your other AR's. If you don't reload, a 300 BLK won't be as cheap to feed as a 9mm. Then again, neither is a PS90. I'm not sure what your home situation is (i.e. house vs apartment or urban vs rural), but a 300BLK can be just as effective at 100+ yards as it is at close range, depending on bullet selection. Not saying you should shoot at home invaders at 100 yards but, if you live in the country, you never know when you might catch a coyote running through the yard.