My wife recently bought a Mosberg 20 gauge over/under. It isn't for defensive purposes but for hunting and shooting trap. A 12 gauge was too much for her.
She also keeps an XDM 9mm in her night stand equipped with a flash light and laser.
I like the 38 special, but I'd recommend a 357 mag. You can shoot 38 special in it too. Just a little more versatile.
I have a Ruger Blackhawk in 357 mag, 38 special. It has a cylinder for 9 mm too. I also have a Winchester 94 Trapper chambered in 357 mag. It goes along good with the Ruger single action.
I like having some of my rifles match my pistol calibers too.