Which of these would be a better choice for cqb out of a ~8-9 inch barrel? For example, a mp5a4/ump9 firing 9mm rounds vs a g36c firing 5.56 NATO's. Plz consider terminal ballistics, accuracy, recoil, etc. Disregard noise, effectiveness w/ silencer, and cost/availability of ammo. I was wondering about the pros of a 9mm smg with a ~9 in. barrel vs. a g36c (also 9 in. barrel ) firing 5.56 rounds.
Given your only 2 choices, I'd say 9mm, with a 11.5" barrel length the velocity of the .223 drops to .22 WMR level. The local Tactical unit uses UMP .40's for entry weapons and 16" barrel .223 AR's for perimeter security/patrol rifles.
My biggest question would be why a 9mm for the pistol caliber, instead of a .40 S&W or .45 ACP and why not a 6.8 SPC or 6.5 Grendel for the rifle caliber?
I would opt for the 9mm because a 5.56 will reach about 2100fps with a 9" barrel and I dont think that is fast enough to fragment the round. They both achieve similar ft lbs of energy but IMO If you loaded your own ammo you could make the 5.56 sbr better.
A 9mm will get a healthy boost in velocity from a 9" barrel. How much depends on the speed of the powder but as above with slower powder you could probably push a 115gr bullet into the 1500's.