I think the hostility comes from people having to believe that they're right and feel superior to those who argue against them. I use to be a big believer of the .40 years ago, but have come off it now when it comes to the debate between .40 and 9mm; now I think they both have their place and reason to exist.
It's .45 ACP that I have trouble justifying owning if you don't also own a suppressor. There is a recoil difference between 9 and .40, but IMO, there is no difference in recoil between .40 and .45, they both feel the same. The difference is you can get .40's that hold 15 rounds while most .45's (Glock, Walther, and FN .45's aside) hold only 10 because of all the BS state mag capacity laws.
I'm not obsessed with capacities when the differences between calibers is 1 or 2 rounds, at least not in full size pistols that hold 15 or 17 rounds because mag changes take seconds, but when the difference between .40 and .45 is 5 rounds, 7 rounds with 9mm and .45, that's significant.
Then when it comes to the single stacks where you are limited with the capacity of magazines I think it's best to go with the 9mm as it offers the highest capacities. Besides, the .40 will beat a poly framed single stack to death quicker, if your hand would allow you to shoot it that much.
Anyway, I own both .40 and 9mm, only .45's I have are revolvers. When I was big into .40 and bought my .40's, I was buying a full size Glock and a Kel Tec Sub 2000, so small size for conceal carrying wasn't what I was looking for. My opinion is that if you are going with a big pistol, buy a bigger caliber, if you get a small pistol, buy the smaller calibers. All these .357 revolvers that I'm seeing that hold 8 rounds because they're built on .44 Mag size framed revolvers... it's one of the dumbest fads currently gripping the revolver world.
Sure a full size pistol in 9mm is the same caliber as your carry gun, but that doesn't make it a better gun than a .40 because when it comes down to it, a .40 will always propel a bigger, heavier bullet faster than 9mm can. What situations does that advantage come into play and tip the scales vs the disadvantages? IDK, but I like the extra capability.
Speaking of disadvantages, what are the tropes people often repeat with .40 vs 9mm?
1. Recoil- As I said, there is a difference in the recoil. People often say with .40 it's a "snap", I've found it to be more of a "bounce." People make you think .40 is near going to break your wrist vs 9mm and I think that's people who don't shoot enough or have poor technique. I shoot a Glock 27 and while I can't put 10 rounds into the target as fast as I could with 9mm, I'm able to put enough holes in the target fast enough to be effective.
I have thought about buying a 9mm barrel for the G27, but I don't think the recoil will be that much less to make it noticeable.
2. Firearm lifespan- Another one is that .40 kills guns faster than 9mm because the frames are built for 9mm, not .40 stresses. That's true, but the length of service of the guns depends on the amount of times shot and if you're not shooting 1000 rounds a month, the .40's will last just fine, at least long enough for you to get a lengthy amount of service from them.
I don't think the small single stack .40's will last long tho if you're putting a few hundred rounds a month thru them tho. They're not meant to be shot much, not even the 9mm's are meant to be shot much.
3. Ammo price- This is the big one people focus on and while target/practice ammo in 9mm can be found for $8/box of 50, the defense ammo is generally going to cost you the same for the premium stuff (HST, Gold Dot, PDX1, etc.) but for the non premium JHP ammo, I've found the Winchester Bonded .40 (same ammo the FBI used) for a few dollars less than Winchester or Remington 9mm non bonded.
Most of what people shoot tho is FMJ and the savings in price of 9mm vs .40 or .45 is significant, to the tune of $100 less for 1000 rounds and that is the reason I started buying 9mm guns recently.
OP, good choice on a steel frame .40 Kahr, I'm looking at getting one used as they're under $400.