I have in several strings referred to the excellent book by Urey Patrick and John Hall:
CAP - In Defense of Self and Others . . .: Issues, Facts & Fallacies -- The Realities of Law Enforcement's Use of Deadly Force, Third Edition (9781611636826). Authors: Urey W. Patrick, John C. Hall. Carolina Academic Press. Even though directed at LE use of deadly force, most of those are simple inane defensive shootings. The general information is excellent and very valuable.
Anyone who owns a firearm and even remotely considers it a defensive weapon needs to buy, read, and re-read the book. There is a whole chapter dedicated to terminal ballistics, and that chapter alone is worth the price. Likewise, one should also track down the writings of Dr. Gary Roberts, the heir to the founder of modern terminal ballistics research, Dr. Fackler.
Placement first. One simply must know the proper areas to target on a human body when it becomes appropriate to shoot an assailant to defend yourself or others. Many targets result in and reinforce awful round placement, usually too low on the torso. From the front, draw a horizontal line across the chest at about nipple height, two similar vertical lines through them, and a horizontal line across the bottom of the neck/throat. That is where all the main blood vessels are, and with adequate penetration, the spine is also in there. On the head, it is actually a face shot; a triangle from the bottom of the nose through the outside corners of the eyes. From the side, roughly straight into the ear canal. From the side, you say? Darned right. It is amply foreseeable that one might engaged in an encounter in which the target presented is not straight on; while more likely in LE, it is not impossible in civilian encounters, and one should at least be prepared for the possibility. (Which is one of the reasons for the FBI standard - the torso target area could easily be blocked by an arm or other intermediate barrier.)
Once placement is sound, then there must be adequate penetration. The example of the deer and oxygenated blood applies to people, too. While few are as motivated as a wild animal, if one looks at the detailed reports about the 1986 FBI firefight near Miami that started this research, the will to fight in the offenders kept them going with fatal wounds for long enough to kill or cripple several agents.
Handgun rounds suck. One needs to be able to shoot until the assailant is perceived to no longer present a threat. This might be one round, or as many as 10 or more. Caliber is not that big a factor in performance, if it is at all. Training to perform to the necessary level is, and among the typical semi-auto service calibers, the 9mm is so much cheaper that smart agencies that care about training are moving to it.
In .38, I have gone old school - standard velocity SWC. Same with .41. In auto-pistol calibers, regardless of the platform from which it is launched, one of the rounds recommended by Dr. Roberts. I would not carry or allow anything that is round nose - that is simply ill-advised to the point of stupid with possible exception of .380. If I really expect a problem and can't be elsewhere, the answer is a long gun. Slugs in 12 gauge, or one of the proven runs in 5.56X45. I do not buy 3rd tier firearms or ammo - I buy proven quality (BCM ARs, Glock pistols, S&W Revolvers; Black Hills ammo for most uses, and a lot of it at a time.) I don't waste money on a new car (my 163K Subaru runs fine and has been paid off for over 5 years), a boat, golf, or other counterproductive silliness. I hopefully will never need the gear I buy, but as a cop and now retired, it is either pointless or priceless - there is no in-between.