You are correct about rarity but in quality ammo (not wolf which it seems so many ARs cant shoot) 1000 rds of 5.7x28 can be had for less than 1000 rounds of 5.56. Now reloading may be a different ball game as it seems that the 5.7 may not be friendly to reloaders. The PS90 is a very fun gun but its cost is inhibitive for some folks. Not only is it expensive mags are too. Like all guns it has trade offs and excels more in some areas than others. I beilieve it is not appropriately compaired to ARs etc because they were made for very different roles. It is a weapon that I find it very easy to put a lot of rounds on target very quickly with. It is not my main HD weapon but I wouldn't feel too bad if all I had was the PS90 with a topped off 50 round magazine. In many ways it makes more sense to buy an AR before buying the PS90 but I dont think you would ever regret owing the PS90 either. I would say the best thing to do is clearly lay out what you want/need and what are the priorities and then adress them as funds allow.