>>27746518SBR AR15 all the way--
Target isn't wearing armor? At CQB ranges and a bit beyond when it comes to a sub-12 inch barrel 5.56 is going to ruin your day. Ammo designed to fragment is still going to fragment even with the velocity loss when it comes to CQB ranges.
Target is armored? If its soft armor, doesn't matter 5.56 is going through. Plates? Ok congrats you stop a few rounds. Doesn't take long to take one to the head however.
With an MP5 your range is limited from the get-go. Armor piercing 9mm exists, but I don't know how much I trust its ability to consistently defeat soft armor. Then again, its hard to come by solid info on the topic it seems. So perhaps I am underinformed to comment. So you want something good for people? With 9mm thats going to be JHP's. No JHP's are going through soft armor. Want something good against armor? Its gonna be a FMJ of some type despite core differences. As such, it isn't going to do *comparatively* much to what is behind that armor.
TLDR 5.56 outperforms 9mm for just about everything at CQB ranges. Has velocity to defeat body armor and also fragment, whereas 9mm you have to choose between people stopping JHP's or AP ammo (that may be dubious in terms of effectiveness...) that is going to make not great people stoppers
