Theres a saying in the industry, "Ametuers talk hardware. Proffesionals talk software."
Most of caliber choice is all opinion. If there was one wonder caliber that did everything, everyone would be using it. First and foremost, handguns suck at killing people. Dont believe that take a tour of detroit metro hospitol and see how many peopl survive pistol wounds.
Wound balistics are a complex field that has so many variable its not funny. Modern ballistics are so good that there is really only small diferences in between the 3 most popular defense calibers, 9mm, .40 and .45 acp.
What we start looking at is not only the balistics of the caliber but what it offers us in other aspects.
For me the 9mm is the best all around defense caliber and here why.
Round count: Round count is almost number one. We have seen time and time again .22 caliber pistols killing someone in one shot and someone taking 20 rounds of .40 and still trying to kill officers. Rare but it does happen. The point is we dont know how many rounds it may take to neutralize a threat. Taking on an EDP, intoxicated, or multiple threats at one time I really would feel more comfortable with my g17 and 15 rounds than my g36 with 7 rounds.
So, usually a 9mm will offer more rounds in the pistol. Hopefully also reducing the chance of having to reload, which takes you out of the fight for sometimes seconds at a time depending on skill level.
Recoil Managment: Anyone that actually knows defensive shooting will tell you the most important thing is to neutralize your threat ASAP. And you do that by putting as many holes in it as possible as quickly as possible. Now, YOU may be able to shoot a .40 cal faster than ME but YOU can put more rounds on target faster with a 9mm than YOU can with a .40 cal. Like I stated before in a multiple target situation you would want to shoot as fast as possible to gain more hits on multiple targets.
Cost and availabiltiy: Can you find the ammo to shoot and train like you should to be able to protect yourself and loved ones. 9mm is cheaper and easier to find than most other calibers there for i can train more with my money.
Like I said these are my personal opinions and should be treated as such. reguardless of what you have train with it and train often.