OK, now we're in my neighborhood! I once did a 20 minute presentation called "The .40 Smith and Wesson". I can tell you all about it!
Of course, that was 20 years or so ago, and memories, well, ya know!
Anyway, here's why the .40 S&W was so popular: Back in the day, 9mm was viewed as "anemic", mainly because of the Miami-FBI shootout. Deserved or not, that was the view. S&W came up with the idea of a .40 cal cartridge that was the same overall length as the 9mm, and so could fit in the same frame-sized guns. (As an example, Beretta 92 and 96. We once took a barrel ONLY from a 9mm 92 FS and installed it in the 96D; it functioned perfectly! Even the .40 magazines held and fed the 9mm ammo)
So, we get a .40 cal that will fire a 180 grain hollow point at around 950 fps, or very close to the 185 grain .45 ACP loads that were so popular at the time. ".45 punch in a 9mm package!"
I can't speak to pressures, but I can tell you that the .40 is heavier recoil and rougher on guns than the 9mm. The only two I've seen "blown up" were traced back to faulty ammunition (the brass case was formed from a 9mm size brass pellet rather that the correct .40 S&W sized pellet). Remington owned up to it and paid for the guns. I have seen several cracked aluminum Beretta frames, though.
As 5of7 correctly notes, bullet technology has advanced significantly since then (tho 25 years ago I never felt under-gunned with 9mm 124 grain Hydra-Shok +P+!), and the 9mm is recognized as a better stopper today than in years past. Many departments are transitioning back to 9mm (cheaper ammo, easier to shoot and shoot well).
Personally, I like .45 ACP for my CCW, but I can't find fault with 9mm or .40, so long as you choose quality ammo and your gun is reliable with it.