I feel for ya Garand...I have only the M&P9, don't own a 40, and I love (read, "LOVE!!!!") 45 acp. Not just because it's a fat bullet, but I like the way the round shoots and feels. My 45s are a 1911 and the newly acquired Springfield XDs.
On caliber, I think 45 is a near-perfect self defense round. 9 is not, but it's cheap and you can cram more into a carry gun, and 50 bee stings is better than a bite from the Rottweiler you left home. 40? I'm not sure FQ is right in that 40 has some pretty damn good ballistics if you read up on it, especially in an expanding round. But for the price you might as well shoot 45.
So I guess I'm writing down all the stuff that's running through your head. I'm a 45 guy at heart and the smallest M&P you can get in 45 is still not very small...a good, all around pistol. The 9 I have is a great shooter, though, very accurate and if you are buying ammo you get almost 2 for 1 against 45. That's a consideration.
One more thing. While your life depends on that weapon the chances of you having to use it for SD are probably about the same as being struck by lightning...maybe less. Carry what you like to shoot, because you LIKE it.