The main thing to keep in mind, in my opinion, is what would you want to have in your hands when the crap hits the fan?
It needs to be generic, a rifle that can handle almost any locally possible situation you throw at it. So, if you live, say, in the Texas Hill Country [like me], you're going to need some range, but also some CQB ability for the towns around here. Hence my choice is something in .308 [currently a FAL, soon to be upgraded to an M14 EBR/Mark 14]. It may be a bit heavy and bulky, but with the hills around here I would need some range for when the
Soivets Mexicans Nazis Chinese Zombies attack. It may have a little recoil, but nothing I can't handle.
However, ladies tend to be of smaller frame and tend to dislike recoil more than men [yada yada yada yeah I know I'm a sexist bastard, tell me something new], and thus need something that fits their needs better. When the crap hits the fan,
you shouldn't be armed,
your whole family should be armed. That includes your wife and any kids old enough to know how to fire a rifle. And they need to be armed with the most effetive weapon they can still practically use. You never know when one of them will be seperated from you, they all need to be able to defend themselves.
In your case it sounds like your wife needs a light rifle/carbine of some type. Mine is the same way. You have a few options in this case: AR-15, VEPR in 5.56, M1 Carbine or [for a really high price range] the new XCR. You seem to already have this covered with your AR-15.
So now the problem becomes one of what to arm you with. Currently you'd be stuck with the M14, which you don't seem to be fond of. You could go with a full-size AR, this has a lot of advantages. For one thing you and your wife would be using the same ammo to kill the Zombies. You can also carry more ammo, fire more rapidly, mount more junk on the sides and generally mod the crap out of that rifle.
However, some things need to be considered first.
For one, do you need the range that the M14 provides? If the crap hit the fan tomorrow and you had to fort-up in your home, would the extra range of an M14 be a big advantage when picking off the walls of zombies rushing your home? Or will you probably not notice? What if you had to flee to another area, in your local enviroment [anything within a 50 mile circle of your home] would the added range be that useful very often?
What about stopping power? Do you have faith in 5.56, or do you want more 'ummph'?
If you don't see the need for the added range or stopping power, then an M14 isn't for you, and trading it in for an AR-15 or another rifle more in line with your needs would make sense.
And since you don't seem to like the M14, and seem to like the AR-15, it only makes more sense.
For me, I'll stick with .308, it fits my needs better.
Just a n00bs advice, your mileage may vary.