Funny you should say that because I like the Glock 19. I've owned the ever reliable wheel gun S&W .38 Special snub and a lovely .380 Bersa Thunder. I ask my gun friend what's the purpose of higher power caliber and what's effective and he explained things like static shock, amount of damage and blood loss (trauma force).
Your buddy is exactly right on the caliber side, another advantage is that it transfers most of it's force at impact so if a shot goes errant it is less likely to cause collateral damage through a wall or other object as a rule, of course that depends on the muzzle velocity of the round. I actually like the Glocks quite a bit, my problem is that I'm a stocky guy and thus tend to bump into things quite a bit, the last thing I need is an accidental arm because I hit a table or wall etc. It's a good function for a duty weapon however due to the need for quick arming if ambushed or other speed requirements, but for me it's a good way to get hurt. My own firearm, is a .40 cal. Baby Eagle, it's got a slide safety which I like because my thumb is within the perimeter if god forbid I should ever be forced to use it.
Placement and accuracy of course are preferable but he told me in real world situations targets are moving, behind cover, at distances and often in low light.
Absolutely, as well it's not target shooting and your own adrenaline is at play as well, this makes it even more difficult to place a shot. For that reason for firearms training most LEOs and military are trained to stop by shooting center mass(biggest target).
He said most popular calibers are above .32 because of these reasons and showed me a demonstration of an 8lb roast being shot by different calibers. Everything above .22 put nice holes in the meat but when it was hit by the .357mag it was split in two, then when the .44mag hit the thing jumped off the table and was shredded beef. He's fired a .454 Casull, .50 Desert Eagle, 500 S&W Magnum and Colt 45-70 Peacemaker. These things feel like 8lb shotputs in your hand and are hard to hold level without shaking. He told me recoil, weight and concealment are reasons the big guns aren't as practical.
I like heavy guns because they don't recoil as much, if I would ever need a second shot I'm not exposed due to ride up, heavy isn't necessarily the caliber size, and your friend is exactly right, big guns are more for show than defense(as a rule) because for example; the D.E. .50 will put your arms over your head if you don't have a handle on it and even with the best stance you have to re-align the shot, that will get you killed if the attacker takes the initiative of firing at will. The big guns are the best though if you hit, it will stop most attackers, the worst to get shot with........small load .22, it will bounce around in the cavity it hits(Skull, chest, groin(ouch)) and tumble around tearing soft tissue, it's one of those "hope it kills you" rounds because surviving will hurt like hell.
Then he went into barrel length, accuracy and all kind of other variables. I ask why did cops switch from the .9mm's to the .40cal and he said for stopping power. He said even though a .9mm w/ 5-6" barrels and hotloads of +P+ to +P rounds can drop a drug maniac it takes a shorter barrel and lighter piece in the .40 S&W to equal. He said now the .357 Sig is getting some attention and I always get an education from him. He told me his Browning Hi Power was Belgium made pre some year, not the Hungarian FEG.
I personally don't trust my life to a 9mm, it's a high velocity round which means it transfers little to no force using a typical ball/parabellum round, hollow points do fix that problem because of mushrooming/fragmentation upon impact, but then again my "range rounds" will accomplish the same thing, then if I do go hollow the advantage is magnified. It's really all about comfort though.
Anyway, I can see the danger in mishandling dangerous weapons and all activities come with inherent risks. There's too much political correctness involved in law making now days and less common sense
. Anything can be misused and as to political correctmess, there was a program years ago that addressed darwinism and man. It was hilarious and sad at the same time, the host basically pointed out that we protect the stupid using law and as a result they breed and we continue to multiply stupid. (I am paraphrasing but that was his basic drift)