Re: MSAR VS. PS90
I've got both. The PS90 is great fun. It's also a fantastic gun for wives, kids and new shooters. It's easy to operate, very compact, reliable as hell, fully ambidextrous, fairly quiet and has minimal recoil. I'd be comfortable trusting it for home defense. Putting multiple rounds on target with speed and accuracy is a breeze.
Fair warning......if you let a bikini clad woman shoot it, the downward ejection will probably leave a burn or two on her sixpack or thigh (or worse). If you smell burning nylon/spandex/flesh tell them to do 5 jumping jacks as an immediate action drill.
The MSAR is not truly ambidextrous...but that can be overcome with technique and practice. You CAN shoot it from your weak side if you have to.
The E4 version is compatible with mags you probably already have. It's compact...adequately accurate and a hell of a lot of fun to shoot. There's also the possibility of caliber conversions if MSAR ever comes through with them. One of the reasons I bought the E4 was the possibility of 7.62x39, 5.45x39 and 6.8SPC conversions.
I love them both but if I had to choose one it would be the MSAR.