As weird as it might sound, the Uzi either a full size or mini. It's a very space eifficient and quite a reliable design.
I finally had the chance to handle a MP5 SMG a while back...NOTHING about it fits me. Even one with extended controls was uncomfortable. It seems liked selector is just too short. It's also needlessly complicated and a pain clean.
As far as 5.7 or 4.6 and body armor...the threat is either from a developing country (no armor) or will be wearing armor with ceramic inserts that will easily stop either round. I'll take a rifle in the later scenerio and stick with the 9mm in the former.