really the only question is.
what will your wife be using her computer for. for (most) people the mba is powerfull enough to fullfill all their needs. shouldn't kid yourself into thinking you need more than 1.4 ghz and 2 gigs of ram to type notes in microsoft word, or surf the net. and as for battery life, how often will she be away from an outlet? you say portability is not that much of an issue, since she will be not taking it outside the house much. so then it shouldn't be a big deal to keep in plugged in.
when suggesting a laptop/computer to friends I often make the mistake of recommending something that would fit my needs. but then I remember most people wont be worried about photo retouching or video editing.
I've also noticed that alot of the people who complain about not being able to edit videos efficiently or retouch raw images, have never even edited a video before, or retouched a photo, and dont really plan to. but they are in that "what if sometime in the future I NEED to do those things" frame of mind. but in reality they may never need to do anything more than messing around with word docs for work.
or maybe your wife needs something for heavy graphic editing. then go MBP, otherwise. go air, or whatever the cheapest option is.