Just a thought.
Well, since I am not the US I can not contribute to the thread much. However, I have had many MP5s including MP5K and the local made even smaller ones named PK06. I consider it good weapon, but when you take the select fire option out, it is just a BIG 9mm/45 pistol!
Why would want to have a pistol so big. Mag cap? Well I rather go with Glock 17/19/26 and buy couple of 31 round mags. Smaller then MP5 and same cap.
If you want to go with select fire version then every one has their own opinion on the matter. I personally prefer only MP5K or PK06 in the series nothing else since they are too big for the fire power, when you can get Glock 18c in 9x19, or Stichken APS in 9x18- both awesome select fire weapons, much smaller than the smallest MP5.