There was good reason to purchase a Mac desktop for photo editing when they were still being produced with multi-processor G5's. Now that they're using Intel chipsets, there's no significant distinction in hardware platform. As such, it comes down simply to a question of operating system. Most essentially, if there are programs that you absolutely need that are only made for OS X, then by all means it's an obvious choice. I have yet to see any such programs in the image editing world. Then of course there's the issue of OS stability. It's an age-old question. My personal opinion is that OS X is currently an intrinsically more stable platform since it's based on UNIX. However, any real functional distinctions in terms of stability are pretty minimal to the advanced computer user. I'm of the opinion that a well-maintained and well-protected Windows system (good AV protection, regular spyware checking, defragging, and an understanding of how to customize the OS in order to pare down many of its useless background processes) is nearly equally stable. If you take my word on this, then I believe it simply comes down to a matter of personal preference. If you disagree with me, then I think you know which one to go with.
Hope that helps.