I have both and after shooting them side by side I prefer the Shield. You can use the flush fit magazine if you want, but 8 round mags are easy to get and cheap without going through the hassle of getting extensions.
IMO the trigger on the new M&Ps is better than the Glock triggers.
I was a bit more accurate with the Shield but I don't know if that was due to the gun or my preference for how it fit and the trigger.