I have both GMR 13, and MPX SBR. It depends on your preference. MPX mags and reloading can be tricky, as GMR has standard glock mags. GMR has tons of options for aftermarket, as does the MPX, but as far as customization/tuneability, GMR wins all day. Both guns shoot amazing. MPX is more of a maintenance pig, as the GMR will run dirty. I have had to change a few extractor springs on MPX already, as I have done nothing besides shoot and clean the GMR. Just a few thoughts....