I've never understood the folks that think a .45 is some sort of magic death ray that causes instant incapacitation.
9mm allows for a smaller framed gun to have a much higher capacity than the larger calibers, faster followup shots, wide selection of ammo, "plinking" ammo is 1/2 the price of .45 which means more time spent at the range, etc.
I've carried almost every mainstream caliber from .25 to .45 over the years (with the exception of .44 Mag and .44 Special), and there isn't a single one of them that won't do its job if you do yours. These days, what I value more than bore diameter is reliability. If it goes bang when I want it to and hits where I point it, I'm fine with it.
Kahr CM9 in a TJ Edmond Leatherworks custom IWB, and a M&P9c in a DeSantis Intruder (ignore the coat of drywall dust on it, I'm renovating a bathroom). I bounce back and forth between the two, depending on what I'm wearing and where I'm going. The M&P gets the nod 90-95% of the time, though.