Okay, condensed version.

Many moons ago, someone got the bright idea to retrofit 9mm handguns to the newfangled .40 cartridge.

There were a number of problems, the Browning Hi Power conversion was delayed because it didn’t feed one particular load and lost out on a number of sales.

The Smith autos were the first to make big sales but they also showed increased wear from the 9mm guns that spawned them, shades of things to come.

Then the Glock pistols were converted.
The problems started from day one, the previously excellent 9mm Glock pistols started cracking at the trigger pin from the added stresses. A second locking block pin was added which cured the cracking but if one looks at higher round count , 40s you can see where the locking block is torqued up and slaps the inside of the slide.
At the same time, the slide rail tabs have had no less than 4 versions because of torque and wear issues, and mostly with the high pressure, high velocity small frame guns that use heavy bullets. (.40)

Glocks can take an amazing amount of abuse and high round counts, the 9mm model 17 has a number of examples out there with over 100, 000 rounds through them. RWS turned an earlier G17 back in for a replacement with over 300, 000 documented rounds through it. Ir was the gun they tested their ammo with, each batch, each change, it all went through that gun.

What you don’t see is high round counts in the .40s, they losen up too quickly and are worn out long before they get anywhere near the 9mm round count.

Next best are the .45 and 10mm guns, the .357 and .40s follow behind.

When people like Larry Vickers and other instructors/operators choose the 9mm over the .40 there’s a reason, much comes to handling ability. The .40 Glocks are the hardest to handle reasonably, ignoring the full power 10mm G29. Its harder to shoot the .40s as well or as easily.

Conversion barrels.
Some advocate conversion barrels, I do also, however keeping within the parent head size.
For example a 10mm G20 can have a .40 and .357 Sig barrel installed as well as the 9×25 Dillon without stressing the gun. If one wants to go bigger, the G21 slide can be installed with the standard barrel, a .45GAP barrel, a .400 Cor Bon barrel, and others. With another conversion unit one can shoot .22lr and another unit allows the use of .50GI.
One thing to remember is that when converting, the ejector housing block really should be changed to match the case head size, this is part of the issues that people who use drop in 9mm barrels have, the ejector hits wrong and the cases sometimes don’t eject correctly. Same for the extractor, sometimes the .40 extractor works and frabs the 9mm rim, sometimes it doesn’t.
This is why the replacement slide (and ejector) because the parts match the cartridge used.

Gen3 vs Gen4.
One has to remember that the Gen4 upgrades weren’t needed until the , 40 guns started having issues.
They went with the dual spring system ala the compacts to help control the battering and wear. Unfortunately they helped create another issue, certain tac lights and lasers will compress the dust cover enough to bind and slop or stop the slide. Part two of this problem, the added weight of some tac lights on the , 40 is too much for the weak dust cover to handle and the gun malfunctions or actually breaks.

In guns designed for or more appropriately as a .40 to begin, the cartridge can be a good one, but the Glocks are just too lightly built in all reality.

A 1911 or H&K USP for example are built to last in comparison, using the .40.

One of my local officers was at the range last week, working with his G22 with attached light. Bluntly it was too much for him, the snappy recoil. I pulled my G30 out, and using Federal HST he chewed the bull out when he previously was lucky to keep it on a paper plate at 10y.

Again, with good ammo there’s little reason to choose the .40 over the 9mm, generally it falls to what is provided duty wise.

I had one of the first G23s to hit CO. And after some rather extensive testing, it got traded for a 9mm, even back then. And this from a big bore person. Within a thousand rounds, it was showing measured wear.

The next Glock 40 I get will be the G40, a 10mm.
But the next .40 S&W Glock I have will likely be a G20 with a conversion barrel, here it makes a manageable and servicable package,

Good luck.
Go rent and shoot both the 19 and 23, it will help you decide.
The other option?
Have the grip of a G17 shortened to G19 size and get the best of both worlds.