I'd go the mossberg route or remington pump route but maybe even consider 20 gauge. That's what one of my sister's has.
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I'm going to break away from the pack and suggest a rifle, not a shotgun. I understand that the objective is to defend at in-door distances. The shotgun does so admirably. It's also slow to reload, slow to deliver an accurate follow-up shots, and it's a very specialized weapon. A rifle, for my purposes, does what I would require in a shotgun and more allowing for longer range if necessary (better to have and not need than need and not have), quicker reloads and way faster follow up shots. The rifle will require more training to master, but the advantages are worth it IMO. I just flat-out prefer the precision oriented features of a carbine. That said, I just saw a nice Norinco SKS go for $200 + S&H. That'd get my vote over a scattergun.
With a rifle, you really have to look at over penetration concerns, especially if living in a neighborhood with close neighbors.
For someone who probably won't train much if any and will probably leave it in a closet till its needed a rifle is a bad choice IMO, a pump gun would be better for that situation. I have a rifle, but still prefer my 870 for HD work
Pumps are recommended because they're easy to operate, easy to maintain, they're reliable and still provide quick reload.
I'd definitely look into a 20 gauge also. Less recoil may be good for the wife. It'll still drop a bad guy with plenty of penetration too. Just remember, birdshot is for the birds, buckshot is for badguys.
Another option is a .357 revolver. Revolvers are also easy to operate, easy to maintain and reliable. If recoil is a problem, load it with .38 special which will also do the job. I would not get a snubbie for a stay at home gun.