Why is a MAC better for graphic design
15 reasons Macs are still better than Windows PCs

Does CS5, photoshop, and ligthroom run better on a mac? [Archive] - Canon Digital Photography Forums
Tests have shown OSX to be much slower than windows in running Adobe products, usually in the range of 10% slower. Lightroom especially though, since LR2 is limited in the number of cores it can access in OSX, but will access all those cores in Windows.

What are the advantages of a Mac over a Windows PC?
Honestly, there isn’t too much difference as far as using one over the other when it comes to being a computer science student, as you can run virtual box of some kind to run the other or any operating system should you need to. Personal preference and price is what it really comes down to.

Mac vs. PC - Apple Laptops and Desktops Reviews
Macs seem to have a leg up in terms of performance. Still, the majority of Americans use PCs, and PCs still seem notably cheaper.