Windows. More suitable for graphics than and other OS. Because, you cannot use CS6 alone, you may need other graphics software which are more suitable in Windows
I have CS6 on both Macbook pro and Windows, I prefer windows over mac, I've been told that mac is known to be more robust but I have never had any problems on windows, in fact I have bad experience with mac...
The OS is not as important as the hardware that helps these softwares run smoothly, especially since the Adobe suite has been developed to work on both systems. If you have an able video card and enough of RAM, both OS would work great. I have personally worked with both MAC and Windows and found both equally effective.
