I love the look of the p90, it will be the third gun I get if cottoni ever answers his PMs about the pistol I want... otherwise it will be number 2 shortly.
Magazine storage is an issue.. one of the things you don't see in stargate lol.. stuffing them into your vest is not really a great way of carrying them. You'll need a drop leg rig and p90 specific mag pouches to hold them, and they're a bit of a pain in the ass cause they are ridiculously tight to the mags.
The other weapon you aren't considering for cqb, which I think you should cause it looks awesome, is the HK UMP... G&G makes it, they call it the UMG. The mags use the same pouches (or similar length) to the p90 mags though.
As for rareness... I've actually seen more p90s than I have mp5s at the games I've been at.
Both mp5 and p90 are very nice looking guns.. HK knows how to make sexy looking hardware.