Therealkoop has some good points.
I have the M&P 9c. I like it. I wish I had gotten the .40c instead for his reasons: it could be a 3 caliber frame.
I have the M&P .45. I like it. I wish the .45c were for sale in California, but that isnt going to happen, so I can't get it.
IF I could, I might not have gotten the FS M&P .45.
The .40c is the same frame as the .9c, so I know it is VERY concealable. It feels great in my hand. But I haven't shot the .40c, so I can't compare how it feels under fire.
I am not a huge .40 fan, as I like the 9 and .45 recoil characteristics better. But, if this is REALLY for carry and I wanted a .40 or greater caliber, I'd get the .40c, it just carries better and you get at least one more round [.45c = 8, I believe .40c = 10]. Oh, and the .45c seems to be almost exactly the same size as a Glock 19, whereas the .40c is definetly smaller.
For defensive purposes, a well designed 9mm shot into the right location beats a well-designed .40S&W round that I miss the target with, so I picked the 9c.
I just wish I'd thought of the conversion potential first!