9mm vs. .40 S&W, I would feel confident with both because I understand only good hits will stop a threat. A .40 S&W gut shot will be less effective than a 9mm rnd to the vitals for example.

I would shoot both rnds from the same types of platforms like Glock's, Springfield XD's and Beretta's see which cal. you can shoot the fastest with the most accuracy out of similar gun type.

I've owned both 9mm and .40 S&W among others but settled on carrying only .45acp because the 1911 is my favorite platform (the Sig P-220 .45acp is my second favorite) and it keeps things simple ammo. wise.

1 shot drop is only achieved by well placed shot's of a decent cal. in the vitals.