.45 vs others?
 

Are the .45 m&p pistols any larger than the 9mm, .40, etc. m&p pistols as far as slide, frame or anything?

Yes, they are larger. Same as the glocks. The parts are scaled at about the same variances as the rounds... (I.e. the M&P 45 is as much larger than the M&P 40 as .45acp rounds are larger than .40...)

My advice is head down to your local gun shop. Try 'em in hand. With the M&P series, the adjustable backstraps can help fit your hand better whether large or small...

I'm between a new m&p or Springfield xdm. Combat sidearm application. Prob gonna end up being 9mm, so I was just wondering. Thanks!!!

9mm .40 Cal are 4" barrels .45 has 4.250 inch barrel

I love my dept issued m&p40, but the XD(m) series has higher capacity in the 9mm dept, I believe.

Both are fine guns.

One consideration is that there is a booming modular aftermarket for the M&P's (such as Haley Strategic's mag release and trigger upgrades...)

As in "Travis Haley" ?

Yes. His company

Oddly enough it's the other way around. 9mm/.40 are 4.25" barrel. The pro series come in a 5" barrel. And .45 is 4" barrel. Although there is a version of the .45 that comes in 4.5" barrel