That's a tough one. Neither of them have particularly good ergonomics compared to the AR/SCAR/ACR type things coming out in droves lately.
Caliber really isn't much of an issue at this point, since even the more powerful 5.45mm round is going to lose a lot of its punch out of such a short barrel, while the 9mm round is going to gain more velocity out of the longer SMG barrel. Normally I'd say rifles are rifles, and pistols are pistols, but we're in AR-pistol ballistics territory on the AKSU. It's almost too close to call, but if loaded with modern, high-pressure JHP ammo, I'll give the edge to the MP5.
In terms of recoil, normally I'd say the 9mm SMG is more controllable, but in this case the MP5 has a very high rate of fire compared to the AKSU. Also, the delayed blowback system has notoriously harsh felt recoil compared to a gas operated system. It's still close to even, but I'll give the edge to the AK on this one.
I've never felt the stock trigger on a Russian-made AKSU rifle, but here in the states, the 922(r) compliance trigger replacements on AK-style rifles are many times better than the stock trigger on HK style rifles like the PTR-91. BUT, if we're going to non-stock triggers, HK rifles have the advantage of a guy named Bill Springfield and his trigger jobs. Nobody said it was a rule that SMGs have to have crummy triggers, so the MP5 wins this round.
Neither weapon is particularly good for mounting optics. That problem was probably solved earlier for the MP5 than it was for the AKSU, but the so-called claw-mount for the HK stamped receiver was never something that inspired any particular confidence for me. I'd prefer something like the Samson MFG rail forend for the AKSU, so it wins this one.
Ergonomics... like I said, neither of them is particularly good by modern standards. The MP5 safety/selector is oversized and nearly impossible to reach while maintaining a firing grip, and the AK lever on the right side just flat out IS impossible to reach while maintaining a firing grip. The MP5 has an advantage of the charging handle being on the left side, but it's still easy and fast to reach underneath the AK, behind the magazine, and work the charging handle with the left hand. The MP5 also has a big problem in that you can't seat a fully loaded 30 round magazine on a closed bolt, so you add an extra step (locking the bolt open) to the process of changing magazines...... unless of course you download your mags to 28 rounds instead of 30. Which I would. Point goes to the MP5.
The MP5 weighs half a pound less than the AKSU, but the ammo doesn't actually weigh any less. There isn't much difference in the weight of a complete 9mm cartridge, versus a 5.45mm cartridge, so for the same weight, you're not going to be carrying any more ammunition for one, than the other. For the weight though, even though it's largely wasted in such a short barrel, I'd prefer the 5.45mm round. I'd say 200 meters is pushing it for either weapon, but it seems more reasonable with the AKSU.
So, both weapons are basically matched point for point, so now it just comes down to personal preference. I'm going to cast my vote for the AKSU, just because I prefer the way it handles over the HK. That and 5.45 ammo is cheaper than 9mm Luger JHP +P rounds. Don't get me wrong, I love my HK-style rifle (PTR-91), but the kinda clunky feel makes a lot more sense in a 7.62x51 battle rifle than it does a 9mm submachinegun.
Good God 
