Well I like my real-caps personally. Although I do see the limitations of them so I'm getting some mids (68-110 rounds). Mids don't rattle at all.
If you plan on playing milsims, then go and get a box of real-caps so you aren't limited when you're playing (eg. some milsims will have an outright ban on high's and mids, others will ban high's and allow limited number of mids on your loadout)
Mids are pretty good all around I'd say for general scrims and the occasional milsim.
Sorry, I have no experience with P90 mags except I know they're a bitch to find/use and find a carrier/pouch that will fit them. Remember no visible fake rounds in your mags or you will get a letter from the CBSA.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE