OK, so you've decided to take responsibility for your own safety. You have seriously reflected upon the idea, that if you are subjected to an illegal Deadly Physical Force attack, you have the moral courage to take actions to stop the adversary even though these actions could result in the adversary's death. You've selected a caliber (hopefully 9mm or better) and quality purpose built defensive ammunition. You sampled several pistols and have settled on one that is big enough for you to handle and small enough to conceal. Now what?
Define your "Mission."
Yes, I have a lot of time in the military and law enforcement. Yes, I use that knowledge, training and experience as I go through life. I fully understand the the LE world is different from the Mil, and is different from the Civ. Got it. But we can learn from them all. The military is really good at breaking big jobs into smaller tasks. They excel at breaking down a given mission into phases and different tasks. These skills are important so that you don't waste time and effort on something that has no bearing on the overall mission.
What is your "mission?" For the purpose of his article, let's say our mission is to "carry a reliable, ballistically relevant pistol, concealed, everywhere I am lawfully permitted to do so (in compliance with my CPL), with the intent to be able to defend myself against and survive an unlawful Deadly Physical Force attack."
"Ballistically relevant" defensive pistol ammunition must be able to penetrate 12-18" of properly calibrated ballistic gelatin AFTER having passed through heavy clothing. Nothing smaller/weaker than 9 mm parabellum can do this reliably. The bigger the cartridge, the bigger the gun and the harder it is to conceal. The more powerful the cartridge, the harder it is to control. ALL bullets are poor stoppers of determined (and/or nutty and/or intoxicated) human beings. You should plan on having to fire multiple shots very quickly and accurately at a small, moving target (brain, heart or spine) while the adversary is trying to do the same to you.
"Pistol." If you can't get all your fingers on the grip with your dominant hand, the gun is probably too small. You need to have as much of your hand(s) on the gun as possible in order to control it. Revolvers DO allow the fastest resolution to a dead primer in that you simply pull the trigger again. HOWEVER, the "foot print" of the revolver is the same for a comparably sized semi-auto pistol. Semi-auto pistols are far faster to reload and revolvers have dismal ammunition capacities. Simpler to use? Yes. But if you don't care enough to learn how to run your semi-auto well, I suggest you reflect upon your desire to defend yourself with a gun.
You must have enough hand strength to manipulate your pistol. A smaller platform will usually have a stiffer operating spring. If your hands and arms are weak, fix them. In order to prove reliability, you really should shoot your defensive ammunition in your gun at least 500 times, preferably 1000 times to prove it functions reliably. Any more than 3-5 stoppages is cause for alarm. I know that is expensive. Sorry.
Night Sights? I've evolved over time on this topic. I now believe that if it is so dark that you cannot see your sights, it is probably too dark to positively identify who/what you are shooting at and you probably shouldn't shoot. This implies consideration of a light. Weapon Mounted Lights are the standard. At least 200 lumens for a civilian gun. BUT WMLs dramatically increase the foot print of the pistol, making it a little more challenging to conceal. Handheld light as part of your Every Day Carry? Sure. But shooting with a hand held light is fraught with issues-the reason WMLs are a thing to begin with. Lasers can be problematic. If you don't train frequently and properly, they can make you much, much slower. Mini Red Dot Sites? Take a while to get used to, can make you slower at close range if you don't train properly, and you must have Back Up Iron Sites.
"Concealed" holster selection and position is a big deal. Inside the Waist Band (IWB) is the most effective means of concealing a gun. Ankle holsters may suffice for a back up gun but are too slow to get to in a hurry. Shoulder holsters can be very problematic. I discourage off body carry (purse) as the gun is too easily left or taken out of your control.
Imagine that your pubic bone is 12 o'clock and your tail bone is 6 o'clock. For most, a 1 o'clock "Appendix Carry" (IWB)) carry is the fastest and most effective as long as you don't have a giant beer blister.
Seasonally, I will transition to a 3 0'clock holster once the weather turns cold as my coat will conceal the gun. If I use an Outside the Waist Band (OWB) holster, it will be equipped with a retention device to prevent a grab/take away. Truth be told, retention is there just to give you enough time to realize that someone is trying to take your gun away from you so that you can respond appropriately. IWB holsters put "all their eggs" in the concealment concept to avoid detection and a potential grab/take away attempt. I prefer a holster that is secure by my pants belt as opposed to a paddle. The paddle is just a bit weaker.
I prefer the OWB at 3 o'clock method as it is more comfortable than appendix. But I understand that carrying a gun should be comforting, not necessarily comfortable. I use the discomfort of the holster to keep myself in "Condition Yellow."
Pocket Holsters/"De-profilers"? Some times a pocket holster is all that will work. However, this is severely limiting pistol size. For my size, a Glock 43 is as about as small as I am prepared to go.
Leather holsters will usually require some time to break in. They tend to retain moisture and may lead to corrosion. Kydex is the new standard and are generally superior. Always keep in mind that your holster should allow you to re-holster one handed. You other hand may be occupied with say, your phone, a loved one, or it may be occupied trying to plug a bullet hole somewhere in your body. Also keep in mind that when that moment arrives that requires you to draw and go to work, you could be in a weird position. Like on the ground curled in a ball as you are being kicked to death. Can you make the draw form this position?
Your spare magazine (at least one) should be on the "opposite corner" of your body from your gun. Buy purpose built pouches. If you want a hand held light, it too should be stowed near your magazines.
Stowe your ID and CPL AWAY FROM YOUR GUN! Like totally away from your gun. If you have to show your ID and CPL, your hand should go nowhere near your gun. If your gun is at 3 o'clock waist line, your ID and CPL should be in a left breast pocket. That's what I mean by "nowhere near."
"Defend myself" means fighting. You have to be able to put bullets where they need to go, quickly and repeatedly. Potentially against multiple adversaries. Usually at fairly close range. You need to keep your wits ("Combat Stress Inoculation") and be determined to see it through (Mindset). You need to keep your gun loaded (Emergency and Tactical Reloads) and you need to be able to clear any stoppages.
"Survive." It makes total sense to have a legal defense plan in place before the event. It makes total sense to get tactical medical training and to carry some basic stuff to stop bleeding and plug holes. If you live long enough to realize you've been shot, your odds of survival are about 90%. But preventing yourself from bleeding out or suffocating makes sense.
The Mission drives the gear.