Hi TheWander,
Here's my 2 cents.
I own several rifles (Tavor, AR, bolt guns), shotguns (Rem 870, KSG) and pistols, I shoot them enough that I am comfortable using any of them and feel I can shoot them fairly accurately.
It sounds like you may be new to pistol shooting, as well as your wife. As most pistol shooters are aware, it takes a fair bit of practice to hit targets with decent accuracy. Not to mention in a situation where you have intruders entering/threatening and moving.
If I were to decide to put any of my guns into a self defence roll, and also consider that a new to shooting wife may need to use it, my first choice would not be a pistol, it would be (as others have mentioned) a short barreled shotgun with a light attached.
Load a 20ga (more wife friendly), or 12ga with #2 or #4 shot would be more than adequate to get the job done, plus there will be much less penetration through doors, walls etc. that may hit innocent others. Firing 9mm, .40 and .45 in a house will penetrate several doors, walls, etc. and may injure innocent others.
Shooting a shotgun would take much less practice time to become proficient when compared to a pistol. You said you were rural, so practice could be done at home. As we all know, a pistol will mean trips to a range to practice.
Also, in a rural setting, by just changing barrels (i.e. Rem 870) a shotgun can be used for other applications.
If you still want to go with a pistol, I'd suggest one of my favourites, a Glock 17 with night sights and a quality light/laser combo. To avoid wall/door penetration and if you can find it locally, 9mm frangible is what I'd consider using.
If you don't know what frangible ammo is, it's basically compressed metal powder. It does considerable damage to soft objects, but returns to powder if it hits something harder.
Here's a better explanation:
http://www.ar15.com/archive/topic.html?b=3&f=16&t=276075