A buddy is an SOT, so I've got to play with his SMGs a bit
MP5's (in general) - Anyone that's read my Z5RS SBR thread knows I'm an MP5 slut. I love how little recoil the roller locking action has and it's fast and intuitive to use iron sights. It's stupid fun and easy to shoot, at least after you get used to the ergos and lack of a bolt hold open.
MP5SD (w/ KAC can)- Amazing, probably the most fun that can be had on full auto. Very quiet, nice to be able to use regular 9mm, and feels like shooting a .22 and sounds like a typewriter. Very accurate and controllable. We've put easily 20-30 mags through it without a cleaning and still runs like a top. Pretty much everyone's fav gun after they get a mag through it.
MP5K PDW- Tiny, easy to conceal and suppressed well. Usually has a B&T stock on it that's pretty comfy, the Choate stock looks cooler but is too easy to unlock when shooting extended. Accurate, but the shorter sight radius keeps it from being as spot on as the SD. Because I have longer arms it's also kinda awkward to shoot.
MP5/10- Only 10mm I've ever shot, seemed a lot more controllable than I'd expect but they weren't full house 10mm loads either. Bolt hold open is nice, but I'm not used to them on MP5's so I always forget it's there.
MP5K Suitcase - Yeah, I've got a few mags through the Teutonic Suitcase o' Death... man is it fun. And stupid. Even at "you've got something in your teeth" distances, hard to hit shit. A .22 derringer is more effective...
UMP 9/45 - I thought I'd really like them, as they were supposed to replace the MP5. Turns out there's a reason why they still make the MP5. It feels really "bouncy", the blowback combined with polymer frame doesn't run nearly as smooth as the MP5 nor as controllable. Also the .45 mags are a PITA to load up.
P90- Extremely controllable, just seems to almost do small circles over the target center mass on full auto, even doing extended mag dumps. Mag changes aren't nearly as awkward as they look, just make sure to slam it in hard with your fist. Loading the mags feels like a never ending chore, each round requires more effort to load and there's 50 of them to get in there. Feels like shooting a .22, but that's because you practically are- a really expensive .22 magnum, and that's where it really fails for practical purposes. I'd be pretty pissed if someone tried to kill me with one.
AKS-74U "Krinkov" - Yeah it might shoot a 5.45x39 round, but it's an SMG right? Pretty fun and powerful for the package, but with its sights and muzzle blast it's definitely a PDW / CQB type weapon. On full auto it's a bit overkill, but on semi feels practical and easy to get controlled pairs on multiple targets.
HK53 - You'd think it's a pussycat compared to it's older brothers, the MP5 and HK33. Yeah, not so much. Pretty blasty, and not very practical on full auto (like the Krink). Also 5.56 ballistics out of a 9" barrel are less than impressive. Still looks wicked cool.
MP 40 - Holy shit these shoot nice. They're straight blowback, but with a low cyclic rate and a telescoping bolt they're total pussycats. I read about a SOG dude in Vietnam that was a WW2 vet that carried one, and trust me... I get it now. Even with it's wobbly stock it was really controllable and easy to get solid hits.