I don't like high-pressure rounds for a number of reasons. One initial reason is the cost of the ammunition. It's pricey, it's often times in restricted availability, and the additional kick in the wallet will inevitably translate into less training with the caliber that I might have to potentially defend a family member with. Unacceptable. However, it doesn't stop there...
For me personally, there is a noticeable performance gap between the way that I shoot a 9mm vs. the way that I can handle a .40 caliber. When handling the .40- the muzzle rise is much more noticeable, the recoil is far more significant, and the follow-up shot times are (in my opinion) unacceptably difficult and slow. I can already hear someone saying "Well, you just need stronger arms" or some ridiculous statement similar to this... Let's get this two part reality of life out on the table:
I truly believe that you should shoot whatever you perform best with. You want a .40? Go ahead! I want a 9mm.
I don't know anyone who is good with a .40 caliber handgun that isn't better with a 9mm. It's that simple.
Yes... I get it that you're a "great shot" with your .40 caliber handgun. I'd still argue that you are better with the same model handgun chambered in 9mm. If you're not, GREAT! Use your .40 caliber! You asked me which .40 I recommend, and I stand by my recommendation that you get whichever one you can get that is chambered in a 9mm or .45 ACP cartridge. Personally, I just don't like high-pressure rounds.
"But the .40 has better terminal ballistics/kinetic energy than the 9mm."
Okay... Scientifically, you are correct... However, if you are using decent, modern ammunition and you can't kill it with a 9mm, then I'm hard pressed to believe that you're going to kill it with a .40 caliber. In our original example of the FBI shootout in Miami- the FBI was correct that they had armed their agents with an insufficient firearm. However, the answer isn't a unicorn cartridge for a handgun. The answer is a damn rifle. The bad guys are shooting at you with long guns, so guess what... YOU SHOULD BE SHOOTING AT THEM WITH LONG GUNS!
If you can't kill your intended target with a 9mm or .45 ACP- then you don't need a high-pressure handgun round. You need a rifle.
"What if you have to shoot through a barrier?"
The best argument in favor of high-pressure rounds that I have heard is for use in defeating barriers. Yes, the .40 will defeat barriers better than a 9mm. The same is true for the other high-pressure rounds when compared to their counterparts. However, I have a problem with this argument as well...
Understand that "defeating a barrier" is going to mean that you are shooting through something that is between you and your intended target. The vast majority of the time, this will likely be a barrier that you cannot see through. Therefore, it would stand to reason that a civilian might have a very difficult time explaining to a District Attorney exactly why they were shooting through a barrier at someone that they could not see. Impossible to explain? Not necessarily... Difficult? Most likely.
The exception to this rule is body armor. With the attacks in Paris and San Bernardino, the notion that no civilian would ever need ammunition that defeats low-level body armor is out the window. They used to be deemed as "cop killer" bullets- but it's likely that they'll now be known as "terrorist asshole active shooter in body armor killer" bullets. However- neither 9mm nor .40 can defeat Level III body armor. In fact, Level II-A body armor can defeat both 9mm and .40, so I don't see your ballistic advantage of a .40 with even the minimally recommended body armor. There again, you need a rifle! If you don't have a rifle, then you're going to have to figure something else out. Change levels and engage the active shooter in the face/head. Can't hit the face/head? Change levels and engage the shooter in the pelvic girdle to immobilize him/her. Then re-engage them in the head/face. Either way- you're not going to have some magical effect with a .40 that you were missing out on with the 9mm.
Use What You Have With You!
If you're going to train with, carry, and depend on a .40 caliber- that's 100% fine with me. I'm not arguing that I'm a ballistics expert. I'm simply explaining my recommendation of getting a 9mm instead of the .40. Ammo is cheaper and more readily available (therefore you will train more). The recoil is far less noticeable with a 9mm. The muzzle rise is more manageable. The overall performance of the gun is simply better suited for most shooters. However, if you have a .40 caliber and you regularly train with it- great! Stick with it! If you don't have either and you're looking for a first-time investment, I would highly recommend shooting both cartridges before investing in either.