So how do I pick a defensive handgun??

 First you want to pick a cartridge that has enough punch to do the job and is widely available.  Of which we have 380 Auto, 9mm, 45 ACP for automatic pistols and 38 Special, 357 Magnum, and 44 Special and 44 Magnum for revolvers.  Then we want to find a handgun that fits well in our hand.   In my experience the old 38 Special Air Force issue revolvers were miserable to shoot.  Grips were too small and they had been marinated in gun oil for 25 years and were slippery.  The Army 45 auto was much better.  Your experience will differ.  But you want to shoot at least one box of ammo before buying the gun.  Once you have bought the handgun you need to shoot it, at least once a month until you can keep all the shots withing a 6 inch circle at 25 yards.  Shooting two handed.  If you cannot hit squat with it, it won't do you much good.