I own both. I very much enjoy the GSG5 which is an early model, and mine's very accurate. That said, it is more finicky (FTF's) than the M&P and unless you like to and are equipped to "tinker" with the GSG's bolt set-up (tightening screws) and other potentially annoying minor details, the M&P is a much more reliable, trouble-free choice. There's also MUCH more to mod and tons of add-ons for the M&P.