It's historic. Mac's used to be much better for graphics type work and media organisations all went Mac. There isn't really any difference now but media firms still mainly use macs. If your wife wants to exchange publications, etc with these firms then it may be best to go mac.
Traditionally the best Desk Top publishing software was available on Macs, nowadays the software is the same on both platforms.
As covergirl says, "arty" types go for Macs claiming they're being individual, when in fact they're following the crowd.
Now if you asked this question about music editing then I'd say go with a Mac, and earlier this year I'd have said the same about video editing, but it looks like the latest version of Final Cut is causing some problems.
But for photo editing I'd stick with a PC, there's far more free or low priced photo editing software available for Windows
If someone else is paying a Mac is good, but I use a W7 PC with Photoshop CS5 with no discernable difference. I do not think the premium is worth it for a private user. A good monitor is worth having for either platform.
If hssutton reads this , his opinion would be interesting.
The only advantage I can see nowadays is the ability to use Aperture. I've tried it many times on a friend's Mac and I love it.
But Lightroom is just as good (and available for both)And if you're running a photography business probably vital.