The .40 S&W started life as the 10mm made for the FBI who rapidly found that if loaded to initial pressures produced recoil that made it difficult for a significant percentage of agents to qualify with their duty weapons. Instead of insisting those trainees become facile with what is a very powerful pistol cartridge, the Bureau decided to "tame" the cartridge in the interest of no one and the .40 was developed as "10mm light". In essence the cartridge is the answer to a question no one should have asked since the 45ACP can do anything better.
On the other hand, the 9mm was developed strictly as a warfighting cartridge, and as propellant and bullet technologies have progressed over time it has proven to be quite a capable defensive round, and is available almost anywhere ammo is sold which is not necessarily the case for the .40. I personally do not own a .40 caliber handgun, and never will again. The 9mm shoots softly and is easily controlled even in +P form, and the .45ACP is (I believe) more capable than the .40 and is no more difficult to control.
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