I've handled and shot both, and just purchased a used GSG. Several reasons:
a) the GSG will accept almost any 1911 parts and accessories;
b) the GSG is well put together, great fit and finish and is the same size as my Norc .45, just lighter. Good for training with cheaper ammo.
c) Internals. Watched an M&P stovepipe one (Remington bucket ammo), then the slide would not close. The extractor is so soft it bent, and there isn't much clearance in the slide for it. Bent it back into place trial and error by hand. This does not scream longevity.
d) Did NOT like the trigger on the M&P, nor the preponderance of plastic... the GSG feels like a quality European made firearm.
However, if you want to do tacticool drills that include dropping empty mags, the GSG plastic bottoms break easily if dropped on hard surfaces. Upgrades are available, overall that's the only weak part of the GSG mags- they are beefy, meant to mimic full .45 mags better, and otherwise are all metal except the follower.
Just u humble opinion.