I like both calibers, but I do not think that highly of M&P and pretty much dislike Glock. In my opinion, 9mm is much easier to stay proficient with because it is easier on the shooter (recoil) and easier on the finances. But if you are already proficient with .40 S&W, there is little necessity to change calibers. As you said, you already have a gun in .40, so it is simpler to have only one caliber for both guns. The question is, are you willing to spend the money to maintain proficiency; to shoot the gun often enough to not be a hazard to yourself and to others when you carry? I mostly carry with 9mm these days, for the simple reason that it is easily concealed all year long, whether hot or cold. Yes I can do the same with the .40, but it is a little more bulky. Neither of my daily carry guns are single stack, although I have a 1911 in .45 that is concealable if I wish to carry the extra weight, which I seldom would consider.