Tossing my hat into the ring, here.
First, let me state that ALL pistol rounds have horrible performance. To achieve optimal "threat stoppage" power, one needs some sort of high-power, optimally big-bore, cartridge. This holds a decided disadvantage in a defensive pistol, as the recoil produced would preclude any sort of timely second shot, unless you've got forearms the size of Andre the Giant's waist. Arguments have raged, both online and off, about the ideal power and performance of cartridges intended for hunting even deer, many of which are far smaller than your average human. The .223 Remington, darling round of AR enthusiasts, is verboten for hunting anything larger than a coyote in many states, yet carries far more ft/lbs of energy than even the vaunted 10mm.
Looking at the difference in muzzle energy, then, the differences among 9x19mm, .40 S&W, and .45 ACP are negligible. From personal observations, I can attest that pistol rounds, even the vaunted .45, hold no guarantee of efficacy; we had a "gentleman" brought in via ambulance one night, bleeding from multiple pistol wounds, including one to the head. Not only was he very much alive, he was cogent and coherent. While a pistol round certainly CAN immediately incapacitate someone, even kill them, it takes far more precision, and reliance upon luck, to do so. Less difficult is killing them eventually (as this fellow mentioned would certainly have died without medical attention), but for purposes of self-defense, this is not a desired outcome; an attacker who is fatally wounded, yet still mobile, can still injure or kill YOU.
I heard somewhere (unfortunately do not have the study handy to back this up), that some 80% of people shot with a handgun survived, while an equivalent percentage of those shot with a long-arm died. While the percentages may not be accurate, the fact remains that, to some definitely visible degree, pistol rounds are largely ineffective when compared to rifle/shotgun. One may make the corresponding, and to my thinking, accurate, assumption from this, that shot placement with a pistol matters far more than whether it carries 500 ft/lbs of energy, or only a "measly" 450. Additionally, one may make the argument that either a larger bullet, or rapid application of additional shots, is more preferable than simple numbers, although there IS a large degree of personal comfort with the selected sidearm, its recoil, and its manner of operation (comfort and ease of use being directly related to reliable shot placement).
So, the "caliber war." My PERSONAL preference is against the .40, with .45 ACP being my first choice, and 9x19mm coming in a close second.
Truthfully, with FMJ ammunition, the differences among the three become more evident, as bullets in this configuration are not designed to expand, but rather, usually penetrate, and "stopping power" is regulated by the original bullet size, and what it impacts. However, with quality defensive ammuntion, this gap gets smaller. The 9mm to .45 gap is still evident, but the supposed chasm between .40 and 9mm, or .40 and .45, is quite small.
So, why don't I like the .40 S&W? In terms of bullet performance, with modern ammunition, there isn't that large an advantage to it over 9mm, nor is much given up by going to the .45. All three are excellent, within the limitations of the fact they ARE pistol rounds. However, I find the .40 to be a far snappier, harder to shoot cartridge than either of its main competitors, giving up ammunition capacity to the 9mm, and lacking in terms of sheer bullet size when compared to the .45. What it comes down to, is that I shoot both .45 and 9mm far better than I do .40, trusting either the heavier .45 to do more damage and thus stop the threat, or the 9mm for its ease of rapid and accurate fire, placing two shots where a .45 might only require one.
For me, the .40 has all the "reset time" of the .45, possibly more, with neither the advantage of greater bullet diameter nor the ease of a second shot. Recoil is quite snappy, and an extended range session with it wears my hands out faster than 9mm or .45. Given my druthers, I'd likely carry either a Springfield XD 45, or a CZ 75 in 9mm. However, I carry a .40, and own several more, due to the fact that .40 is required by my agency, so as to stay reasonably accustomed to it. But I doubt I'll ever be quite as comfortable, nor as proficient, with a .40.
To sum it up: If the .40 is what you're most comfortable with, by all means, carry it. If you prefer 9mm, or shoot a .45 more accurately, carry that. How YOU shoot your pistol of choice is far more important than the numbers; if someone carried a .25 ACP because that was the gun they could guarantee they'd hit their mark with every time, I'd not want to ask them to demonstrate on myself!