If one were to be forced to knowingly go into harm's way, he would want to carry a long gun because rifles and carbines have much greater power and they literally do drop people in their tracks.
That is one reason, but maybe not quite the reason it's made out to be. Rifles and carbines are much easier to hit with at distance, and they tend to be thought of as easier for less trained people to hit with
quickly even at shorter ranges. They also carry more ammunition, generally, between reloads. Some of those things might be important for peacetime defensive purposes, but most really aren't.
As far as dropping people in their tracks...maybe. Maybe not. Just like with handguns, shot placement and the vagaries of physiology have huge effects. We have enough returning vets who've been shot with rifles and who've shot motivated bad guys that the the myth of knock down power should largely have died by now.
Now that said, we generally look at very high-powered handguns (e.g. 44 magnum, etc) is being over-powered for self-defense purposes.
Sure. Several reasons for that.
These two arguments seems to contradict one another. So in your opinion, what gives?
The quest for MAXXX POWR!!! is always tempting, especially to less experienced shooters. Very much in the same line as all the folks who buy endless strings of the latest and greatest new gun gear. It's an indicator of less developed understanding of the practice and the issues involved. Trying to buy proficiency through "stuff" or "firepower."
Handguns work as effective defensive tools when a) they are present when needed (thus small enough to carry conveniently, b) the user has good skills and maintains them (much practice, and as much training as time and finances allow), and c) they are employed quickly and repeatedly as needed (fast repeated shots until the threat is down).
Magnum power really isn't a factor here. "One-shot-stops" is a largely worthless measure that does more harm than good in sorting out what to carry for defensive purposes. What matters is what gun(s) can you carry every day, shoot very accurately, very quickly, and develop the most proficiency with?
While we intuitively surmise that we are not aided much by going "too light" (.22s .25 ACP, etc. are poor choices), only with a decent amount of experience can we determine where "too heavy" is, and that's arguably even more of a problem.
The more power a gun's cartridge puts out (generally speaking, of course), the larger and heavier it has to be, the more the practice ammo costs, the harder it recoils, the harder it is to shoot well, and the harder it is to shoot accurately at great speed. Most shooters can learn to handle a .38, 9mm, .40, or .45 very quickly and accurately in at least some types of handgun. Beyond that, effectiveness starts to fall off.
Picking a common service sidearm, firing a common cartridge, is real wisdom. We know the rounds WORK for defensive purposes. There is no clear evidence that any one cartridge stands way out beyond the rest, so pick one you like and set the question aside. Then get training, train with others, and practice, practice, PRACTICE.