Many pistol advocates would say a shotgun is not as effective as a handgun for home-defense for three main reasons: difficulty in turning in tight spaces, shell capacity, and recoil. However, these reasons don’t usually apply to home defense.
When defending your home, it is rarely necessary or advised to move throughout the house, and even when you need to, civilian homes very rarely have such tight hallways or spaces that turning around is an issue even with a shotgun. Regarding shell capacity, the suitable shotgun will hold enough - the legal limit for handguns or shotguns in California is 10 rounds, so reloading will likely be necessary for either.
The third problem brought up is recoil management. A 12-gauge does have a pretty good kick. Depending upon one's age, gender (strength of individual), health (such as arthritis), or other physical challenges, the shotgun, a 12-gauge, may not be the best fit, but a 20g might fit the bill. The blast from a shotgun with the proper self-defense ammunition will stop a threat; whether you go with a shotgun or handgun, it's your choice to make based upon your circumstances and health.
