For a handgun, self defense: .40 cal is good choice. 9 mm also good. Both need quality ammo, and preferably JHP (jacketed hollow point) for good expansion. Do not use FMJ as they tend to hit and keep on going...
Shotgun is also good choice. 20 ga is nice middle of road without too much recoil. I wouldn't use anything smaller than #4 shot; and if you're sure of your target, choose #3 buckshot.
Semi-Auto is excellent choice for ease of rapid shots. Wheel gun takes a lot more pressure on trigger and you only have 6 shots, and usually more recoil. Unless you're very qualified I'd avoid a Glock since they have a unique safety feature that IMO is not "positive" lock.
Whatever you decide on choose a piece, probably that costs in excess of $500. You want want high reliability. Do your research. Best guns IMO are Glock, S&W, Kahr, H&K, Baretta, Para Ordnance, Sig Sauer, and related.
Also, PRACTICE, PRACTICE! With any new handgun, target practice at 7, 10, 15, 25 yards. Minimum proficiency would be around 200 to 300 rounds. Practice regularly.
IMO, any handgun smaller than 9mm (unless you are a crack shot to a critical organ) is just going to make the perpetrator mad (e.g., .32, .25, 22 cal (or) birdshot in a shotgun (unless you're close and hit them dead center with the full load).
It is your life! Buy the best you can and the best ammo you can. You want the piece to go "Bang" the first and every time...