I would stick with a 12 ga. for a shotgun. Even if you are recoil shy, a 12ga. shotshell can be loaded down to equal 20 ga. You will have a much better range of options with the 12 ga.
I prefer the mossburg 835 ulti-mag. It will hold six 2 3/4" shells, or five 3" or even 3 1/2" shells.
Dont screw arould racking in a shell to try to scare someone. Thats a great way to jam a gun in a panic situation. It should have a shell chambered, just keep the safety on untill ready to shoot.
I like a 12 Ga for home defense but wouldnt recommend more than Turkeyshot for the average joe and maybe a 2 3/4 load of 00 buck at the end of the mag