I'll throw my vote in for a shotgun as well. My home protection gun is a short barreled Mosburg 500 pump 12 gauge. It's a fairly cheap gun that you can get almost anywhere (probably even WalMart). I have a pistol grip, but don't have it mounted.
I chose a pump gun over a semi-auto on the theory that the best defense gun should never have to be fired. I figure even the stupidest criminal should know what a shotgun sounds like when it's racked and will hopefully take that as his cue to leave without instigating a confrontation.
Handguns are great for carrying and offer easy maneuverability in tight confines, but you're VERY unlikely to actually hit anything. I forget the numbers, but even trained police have an incredibly low hit ratio to shots fired. How well do you expect an average Joe would do?
Rifles are for hunting. Next to useless for close-range home defense. Long and cumbersome.
Both rifles and handguns also suffer from over penetration concerns. Any high velocity round will punch right through the side of your house and carry on. And on. And on. Being in the country you may not have any neighbors, but at the very least consider the consequences of shooting through a wall or two (just how effective do you think drywalled interior walls would be at stopping something like a .357?) and hitting a family member on the other side of the house.
One last thing, if you're wife will be using it she may find the kick of a 12 gauge a bit much. Don't shy away from a 20 gauge. Better she actually uses the thing and gets accustomed to it than getting the bigger gun that she's scared of.
Get a shotgun, and then spend a lot of time putting shells through it. Best choice, hands down.
Google shotgun self defense. You'll find no shortage of information on the topic.
Don
'01 22 Classic, 502/B1
And a bunch of other stuff
"If everything seems under control, you're just not going fast enough" - Mario Andretti