I've posted about this before, but Federal Premium gives away a cool little ballistics program on their website. It has only their brands of ammo, but combined with penetration tests in gelatin you can find online, it's pretty interesting.
In any caliber, the smaller ammo is hotter at self defense distances, under 10 yards. So comparing the lightest Hydra Shock JHP in 40S&W vs 45 ACP, for example, the 40 is about 200 feet per second faster out of the muzzle, and hits harder out to 15 yards. Even going for heavier bullets, the standard 230 grain 45 ACP is still slower and doesn't hit as hard as a 155 grain 40S&W out to 30 yards.
I'm familiar with the argument that you don't hunt elephants with 22 cal bullets going mach 5, you use big, slow bullets. But the penetration data I've seen shows that 45 and 40 are pretty comparable. And, btw, the Federal ballistics program says the 45 GAP (Glock Automatic Pistol) is much hotter than the 45 ACP (Automatic Colt Pistol). The 45GAP round doesn't seem to be widely used.
I don't think either 40S&W or 45 ACP would be pleasant experience. I certainly don't see how 40 could be a solution in search of problem.
I think the selection comes down to other things. Other people have commented on getting a couple of rounds more in 40 than 45. I find 40S&W is relatively available even during this shortage, compared to some of the other calibers we read about.