Lohe pretty much summed it up. The LSC flat is currently the most accurate to genuine HK specification that is readily available. That being said, you also need the right builder to roll it into a receiver. No matter how good the flat is, a bad roll job can ruin all that. If you can find an MKE flat, that is a Turkish flat stamped with licensed HK tooling. There aren't many left and when they come avaiable, the are quite spendy as they are the closest thing you will find to a genuine HK flat. Do note however, that the MKE flats were imported prior to them making it to final stamping. As such, a lot of work is involved in turing those into a firearm.
If you shoot both the UMP and MP5 (even both in 9mm) they are two completely different experiences. Take your BMW for a nice stroll on the Autobahn; that's an MP5. Now take your BMW for a European off-road excursion; that's a UMP. The bolt ina UMP is nearly the same weight as the rest of the firearm. As such, it creates a great deal of rocking motion, or as I like to call impulse, while shooting.