I agree with Clercon that the most important factor is what the user is going to feel at home using. Learning a new OS on top of learning new software can be frustrating.

Now I personally would recommend getting a mac. (my opinion)
1. I'm biased towards macs
2. You can install parallels or use boot camp if you really need a windows environment.
3. Adobe apps work great on a mac, and the ability to PDF from every app without additional software is a huge bonus IMHO.

But the main reason trumps my personal taste.