If you are only going to have 1 HD weapon make it the 12g/20g pump 20g if your wife/gf is small. I think every home should come with a shotgun. Just like a fire extinguisher. Hope ya never need it but it should be there in case you do.
If you want a handgun also there are tons of choices atm. There may not be next year but right now there are if you can find em. So it really depends on your experience and what you are comfortable with. I would want something with dual purpose such as a carry/hd gun or if you are interested in handgun hunting maybe get something you could use for both. 357 with a 4in barrel is plenty for hd/ hunting <deer in iowa anyway> and carry. I carry one as a bug. There is a large variety of bullet choices and you can get revolvers in 6, 7 or 8 shot. It will shoot 38s for practice or low recoil defense ammo. It is probably one of the most versatile gun/ammo combos available.
As important as having a home defense fire arm is the plan is just as important. Getting everyone to a "safe room" where you can hold up and call the police. Just like a plan for a home fire this should be part of every households emergency planning.