I currently own six 9MM’s, five .45 ACP’s and own no .40 Cal. semi-auto pistols. I have extensively shot all three calibers. I prefer the 9MM as my “Go to” EDC pistol for the following reasons.
Milder recoil 9MM
Quicker follow up shots 9MM
More accurate rapid fire shot placement 9MM
Larger capacity magazines 9MM
Total weight of loaded pistol 9MM
Benefit of modern cartridge design 9MM
Smallest overall package 9MM
Compact design small, light, slim and concealable 9MM
Less muzzle flash at night 9MM
Cheaper ammo, both to buy or reload 9MM
Lower pistol prices both new & used 9MM
More product selection 9MM
NATO round 9MM
Most widely used US service round 9MM
This list is only how I rate the three calibers and what works best for me – your mileage may & can vary. Any of the three calibers will do the job for civilian SD. If they don't - you need a rifle.
We humans come in different heights, body strength, hand size, muscle tone, noise/recoil sensitivity and many other strengths and weaknesses. SD caliber selection is not a “One Size Fits All” situation. It is a very personal choice. I can’t and won’t select for you and I won’t let you select for me.
And we can't neglect the "Law of holes"
Bigger holes work better than smaller holes - .45 auto wins this one - BUT-
Many holes work better than fewer holes.
Quicker holes work better than slower holes.
Accurate holes work better than inaccurate holes.
Head holes work really well.
Shoot until it stops! Dan