Doc, since the primary purpose of ergonomics is to assist the tool in its performance of given function(s), what you're realistically dealing with regarding the M&P is more a situation of comfort as opposed to efficiency.
While I suspect that you may like the M&P more than the Glock, the reality is that you quite simply perform better with the Glock. This is about the time that scientific reasoning should come into play...
To paraphrase someone else (who's no doubt a far better shot as well), this is a classic case of a gun needing to be comforting as opposed to comfortable...
...And just in case I haven't hit you over the head vigorously enough with my verbal tour-de-force, yeah, you need to choose the Glock.
Best, Jon