Kevin 90% of most encounters having to use a pistol for self-defense happen at closer than 10 ft in distance! Most shots fired in a altercation on the street are less than 3 according to the FBI reports. The proper fit for your hand on that pistol is more important than weather it is easyier to conceal, at least in my humble opinion. I don't have any problem finding a way to conceal my Kimber model 1911 or my Glocks with 4.6 inch barrels.
I personally believe the 45acp is the best caliber for this kind of duty, an UP CLOSE "Stopper Caliber" compared to the 40S&W cal. There is just something about that big 230 grain weight bullet making a big hole in what it hits, while stopping forward progress of the "perp". However, do test fire both calibers before you purchase OK. You want the make, model & caliber, YOU can shoot the most accurate and fills the best in your hand period.
You can call it smackdown power, knockdown power, kinetic energy or whatever......this caliber has one heck of a track record dating back to very early 1900's. Two World Wars and numerous other conflicts by our troops in battle have given it a thumbs up in close range battle. The Glock 21-SF with 14 round total is a good choice for me, since this pistol fits my hand to a Tee. I have run tests on most all those pistol calibers used in small revolvers and semi-autos from the .22LR up to the .44mag in wet newspaper and plywood boxes. Also used the "self-defense" loads in most of these calibers. The 40 cal or the 45 is an excellent choice for a CCW weapon or tool on the streets.
We own Colts, Kimbers, Springfield, Smith & Wesson and Sig Sauer pistols, but our CCW (carry self-defense weapons) weapons are indeed GLOCKS!!!