I will be buying a S&W M&P pistol.  I was think of the .40S&W,  the 9MM or wait for the .45.  This is not going to be a carry gun.
Here is my logic.  I try to keep my calibers down to a manageable number.  I stash and shoot 223, 5.56, 38 Special +P, 357 Mag, 9MM and 22LR.  
I have shot every cal pistol from 22 to S&W 500 and am comfortable with them all.  I would like to stick with 9MM with the M&P but need some advise on the difference in stopping power between the 9MM and .40.  I use Speer Gold Dot 124gr+P and Gold Dot 147grain in my full size 9s and 124 grain 9MM+P Speer Gold Dot Short Barrel in my compact 9s.
With the constant inprovements in handgun ammunition will the .40 have a big difference in stopping power over the loads I am now using in 9MMs.  It would be great not having to stock another caliber ammo but I don't want to sacrifice a hugh amount of stopping power.  What do you think? Thanks

um i'll stick to 9mm because because I don't wanna go broke shooting and it can be had everywhere.  oh & last i checked 9mm is plenty to kill.  

Overhyped re-worked Sigma.  Saw the guys from S&W on Thursday at a range session for my dept.  Not impressed ....at all.

I have one and like it.

Quoted: Overhyped re-worked Sigma.  Saw the guys from S&W on Thursday at a range session for my dept.  Not impressed ....at all.

And a magazine disconnect for civilian models doesn't make sense either.

I thought it was a reworked Sigma too, until I got to play with one.  I really like the M&P.  From what you've stated, I would stick with the 9mm.  Run decent ammo as you already seem to be doing and it should be fine for defensive purposes.