(Personal Opinion) I've always found the UMP to be "bulky/blocky" and not really all that small. Light though. Mags are loooong and straight so think about what rig/carry system you're going to run. The battery options for it are limited.
I'd repeat those comments for a G36C system (which is actually about the size of an M4 CQBR)...except the mags are wide/blocky.
As alternatives...a PDW is a flexible option...so is a CQBR (or shorter...say Diablo or Tanker) M4. Lots of common/crossover parts with other systems.
A good old standard is an MP5 based model. SD or not...collapsible stock or not...etc...
Just some options to think of