I looked at doing the same thing. The PS90 and FN 5.7 are lightweight and accurate and I like the idea of a combo that shares a common round. The pistol is very high cap and accurate to 100 meters, while the PS90 is a pretty sexy piece of kit, but runs out of gas at 200 meters. The downer for me is that it's a hopped up .22 Mag. Ammo is an expensive proposition (Though what isn't these days). The real deal killer for me though is the construction of these weapons. I have no doubt that they are well constructed and are safe and will do what they are designed to do, but they look like they're made of pot metal. The PS90 is not something that looks like it should cost $1500.
My advice to you is to do what I did and stick to 7.62. If you feel the need for a shorter, lighter piece then get a Socom16. It costs about the same as a PS90 and will also hold 20 rounds.
