I find that the double column magazine guns (Models 669, 6906, etc.), with their corresponding bigger, fatter grip frames, are less easy to conceal than single column magazine guns (Models 3913, CS-9, etc.) with their smaller, thinner grip frames.
If a maker offers a .45 choice, it is usually built on a bigger frame than the frames used on 9mm and .40 pistols, and thus will be harder to hide, as well.
I find that the best concealment 9mm pistol of any make I have tried is the S&W Chiefs Special in 9mm (CS-9). I have one of those as well as Chiefs Specials in .40 S&W and .45 ACP. The two bigger guns are, well, BIGGER.
I have qualified my CS-9 for department use but haven't carried it much yet, except as a third handgun, in my tactical vest (body armor and other gear), and where it would actually end up a forth gun overall after my Colt 1911 Gov't Model or Commander in .45 ACP, S&W Model 642 .38 Special and Colt 5.56 mm M-16.
The sticky, large rubber Hogue stocks on the Chiefs Special pistols make the guns easier to shoot well, but harder to conceal due to their size, and harder to draw smoothly due to their 'tackiness.' Adding a hard plastic set of stocks from a Model 3913, cut down to fit the shorter CS-9 grip frame, would be the best way to set the gun up, in my mind.
I should declare my bias. I am afraid that the 9mm gets little use outside of the tac vest carry. For just plain CCW use, such as off-duty carry for a cop or primary carry as a citizen, I usually carry either a Colt lightweight stainless Commander in .45, or if it seems to be too big for the fashion of the occasion, a .38 Special S&W Model 642 revolver. I am a really dedicated 1911 carrier, preferring 'cocked-and-locked' single action preferable to either traditional double action/single action and double-action-only pistols.
I recognize that many handgun toters are not comfortable with cocked and locked carry. A CS-9 or similar gun, with SA/DA action or converted to a DAO action, is something many are more comfortable with and would be a great choice for that tactical and social niche.
Find and shoot a CS-9. I really think you will like it.