Go to the gun store and handle both the Glocks and the M&Ps.
Which fits your hand and feels the most natural when you bring it up to acquire the sights?
Both are quality pistols and you can't go wrong with either one.
Personally I went with the M&P9 and have been very happy.
But I still have the itch for a Glock 19 I haven't scratched yet.
I have a few Sigs but the P320 just didn't impress me.
