I attended a industry training session two years ago for some of the (then) new stuff from Speer. They came into our our and put on a training session that was 8 hours long. It was both classroom and range time.
What folks need to understand is that, with today's ability to both engineer and manufacture, ammo is FAR more advanced that even a decade ago. And many of you are relying on data and/or experiences and/or rhetoric that comes from FAR prior to that ...
All the major manufacturers are able to design/build rounds that take the entire package into consideration. It's not just the bullet weight, but design. It's not just the design of the bullet, but the propellant. It's not just the propellant, but the caliber. It's not just the caliber, but the length of barrel. It's not just the barrel length, but the intended target material density and depth. ALL THESE THINGS, when properly tuned and considered, make for very good performance; that being defined as both maximum penetration without exiting the subject matter, and having good expansion.
During the range time I was convinced that any 9mm of proper selection will penetrate and expand to a desired level as much as any .40 or .45. While there is admittedly some physical difference between an open 9mm and open .45, that difference pales in contrast to all the OTHER variables that go into this consideration. We shot a lot of ammo into gelatin. We shot though automobile windshields. We shot through plywood and drywall. We shot through layers of clothing. We shot through glass, through clothing, into gelatin ... You get the point. We spend hours on the range and replicated about every reasonable condition one can expect to find in a typical shooting. And in every scenario, the 9mm did what the .40 and .45 did; they all averaged about the same total distance of penetration, all with good expansion. The products today, when properly selected, will do what you want them to do. There is no benefit to size as the sole priority any longer.
In fact, it's getting so specific that they can tune the propellant to the intended barrel length; so that you get the desired velocity from the weapon necessary to get the desired penetration and expansion. Using the same product (same round) in both your full size Glk 17 and your sub-c Glk 26 is not ideal. While any particular round will function in both, it will not function ideally in both. Yes - it's getting that specific in terms of both the engineering and manufacture that ALL CRITERIA (caliber, weight, propellant, barrel, etc) all matter in the production of the ammo. It has even been discussed that there is reasonable thought for having a "summer" and "winter" rounds, due to the expectation of different clothing in some areas like the mid-west. It is truly getting THAT detailed. Sad thing is, most departments are not going to buy four kinds of ammo for their officers (summer for full-size gun; summer for back-up gun; winter for full-size gun; winter for back-up gun). But the point is that the industry can now provide ammo that is so tuned that these considerations actually can make a subtle difference, if you're inclined to practice and utilize it.
The proper priorities of choosing a handgun is these:
1) can you handle the weapon and ammo to a degree that allows confident shot placement under stress?
2) is the ammo able to perform to the desired effect (deep penetration without exit; high expansion ratio) in the expected target material?
I have the control-ability and money to shoot whatever I want; I've got just about every common caliber in a handgun you can imagine. After attending that range demo day, I'm convinced that I no longer needed to carry a 10mm, and that our on-duty ammo (9mm) was sufficient to do what I want it to do.
Now, admittedly there are times when a larger bullet may make sense; typically when we leave the topic of human interaction and introduce larger mammals. And if you have to "compromise", because your risk of encountering a bear or a human is 50/50 while remote camping, then I guess you need to choose whatever makes you feel the most confident. But that simply goes right back to items #1 and #2 above.
Point being this .... There was a time when only size mattered. And then that was usurped by the thought that velocity was paramount; many believed that if fast was good, then faster was better and fastest was best. But that's not the case any longer because size and velocity are only a few of several parameters that go into the entire equation. This topic (that of round selection based on size) is not unlike a lot of topics today. Technology has brought ever better products into the market, due to the ability to design and make the product with ever tighter control. But that ability also comes with a cost. As you engineer ever closer to the edge of the envelope, you lose some amount of "general" performance, trading it for "specific" performance. Whereas we can now select a near-perfect choice for some situations, that same choice will be not the best for others. It used to be that choosing a large round simply made sense because the other parameters were essentially ignored. But the "need" for better performance now dictates ever close proximity to the edge of the performance envelope.
A well-selected 9mm, for the weapon and person, is every bit as effective as any .40 or .45 today. It's a "tuned" package, and when you get it right, it does what it's supposed to do. And if you get it wrong, well it's not going to make you happy when it matters most.
Ironically, the things that are so well controlled today (in round selection) pale to the things that are NOT in our control ... What is the target wearing, how does it present in terms of position that affects total frontal area and tissue depth, what intermediate barriers may exist, etc? It's just as conceivable that any round (9, 40, 45) could either over or under penetrate, causing expansion to suffer. But the last time I checked, we cannot freeze the split-second scenario, walk to the correct position, remove obstacles and clear the background of unintended targets, all while the suspect stands still and awaits our shot(s) ... The things we can now control in terms of making a highly successful round still are usurped by the things we cannot control, and any one parameter you think is "best" is only going to disappoint you as soon as the potential situation changes. And it always does.
If you believe that you're best option is a larger hole, then that's fine for you.
For me, the "best" option is a round I can put on target with great accuracy under stress, while providing a desired effect of expansion and penetration, overcoming any potential intermediate obstacles, while not striking the person behind the person I'm shooting at. The ammo industry is now able to get me ever closer to that desired state. As long as I do my part in proper selection.