You'd lose a lot of benefits of the PS90 platform by going with an AR upper. I know it's cheaper to go the upper route, but the main draw of the PS90 is ergonomics, light weight, and (most importantly) the bullpup deisgn. It's extremely compact.
(Not to mention, accurate, but if the AR upper mfg. knows their business on barrels, that'll be a wash).
Since the ergonomics in the upper are going to be the same as the parent platform (AR-15), you really need to compare what you're gaining or losing against an AR upper. You aren't switching weapons platforms, you are converting an existing one.
Ability to shoot a new caliber? Great, *but* it's one that has historically been kind of hard to locate ammo for at times (now is not one of those times, fortunately, I just picked up 1500 rounds of Fiocci last week and got to comparison shop from vendors with it in stock).
Ability to hold more ammo? Great.. but you can do the same thing in a conventional AR with a beta mag. And... (hate to sound like an anti-gunner here), but you have to ask yourself "do I really need the extra X number of rounds?" With the cost of ammo, you're talking about burning $25 per 50 round magazine. That's not cheap. (Same cost as most 223 ammo).
But I reload...! Well, so do I... and the 5.7x28mm is no joy to reload. You have a dramatically reduced projectile selection over a 223 cartridge, a very narrow band of suitable powder burn rates (powder is still hard to find in some areas), etc. I looked for quite a long time before finding projectiles and suitable powder to use last year. On top of that, the cartridge is a persnickety little bugger to reload. Not at all like other rifle rounds.
Now.. if you already have a pistol and simply want to be able to use the same cartridge to reach out to 200 yards, go for it! (It is perfectly capable of remaining very accurate out to 200; I've won sporting rifle matches with a PS90 before. At 300 yards it's dropping like a meteor though, much more difficult to score hits, but still possible)