Well, it's the MP5 for me. The reasons:
1. As it seems, it is a trusted and proven arm, with a lot of law enforcement agencies worldwide using it. Special military forces use it, too. So, it seems to work.
2. Some years past a former schoolmate of mine joined the police, where he was trained with the MP5. He had been serving with the Bundeswehr and said, if you were used to a G3, you'd have no trouble with the MP5. He is no loudmouth, so I'll trust him with that.
3. The MP5's housing accepts the same sighting devices as the G3. So you could use starlight scopes, telescopic sights and so on. I don't know, if it is neccessary, but it is a cool option.
Some words on the MP2A1 (Better known as Uzi. In this case the variant with the folding metal shoulderpiece.), the only SMG I had to deal with in RL: As the MILAN gunner in our squad, I had the questionable honor to be equipped with one of those. Well, the design plainly sucks. Whenever you were ordered to "shoulder" the weapon and hurry to another foxhole or whatever, that little bunch of junk ended up in your groins. No matter, how you tried to hang that thing over your back, something hit you. No matter, if it was folded or unfolded, magazine or shoulderstock bit you in a very prominent part of the male body.
And: The Uzis, we were equipped with, were so old, Major Uzila has certainly put them together all by himself.
When I first was ordered to shoot with it at the firing range, we were commanded to fire 3 single shots. What happened: I pulled the trigger and the whole magazine load was sprayed towards the target. My Stabsunteroffizier went red in his face, opened his mouth and, before giving me some "further instructions
", looked at the safety notch. Hm, it stood on "single fire". He got calm again and just said: "Oh, it's one of those ..."
Fortunately, our company had too much G3s in the inventory. We (the MILAN gunners) could most of the time choose our weapon. Now guess, what I took ...