I owned a RDIAS at 1 time many yrs ago, and it was mated with a Colt 9mm set up as a Model 635 clone. I've shot MP-5's in the military a bunch, all models...K's, SD's, etc. While it has it's features and such, in my personal opinion after owning 1 for several yrs and shooting thousands of rds thru it with no problems, I still like the 9mm AR. What makes it desireable is that it is the same as M-16's that everybody is familiar with, mag changes are quick, all kinds of bells and whistles can be added on fairly easy, and parts aren't that hard to find or as expensive as the HK stuff is. Even thought the SD model is sweet to shoot, at 1 time Gemtech(I believe that is right) made what they called the Talon SD suppressed upper for the Colt 9mm. Supposedly just as quiet as a MP5SD, it was a nice looking upper. Hope this helps muddy the waters just a bit more!!