I work in design and have only used windows PCs. I think some of these design agencies only use macs, because they look nice on their desks, so is a bit of a status symbol thing. They all run the same adobe software so I can't really see it being an issue as to whether they choose a mac over a pc. Although I would suggest getting a PC with windows 7. As you will be using the desktop there is no reason to go with windows 8 which has downgraded the desktop component, and I find flip 3d and transparency really useful, which is missing in windows 8.
Macs are definitely popular for the image more than anything, at least in this field.
To the contrary I think desktop is exactly the same if not better in Windows 8. Combined with all the other performance benefits, not to mention the 8.1 update, I can't wait to move to Windows 8!
I've got pcs running both 7 and 8 and 7 is my preferred one when running 3 screens on it for productivity use. 8.1 may fix this. But I find 8 is used more for consumption activities, and it is a pita when it flicks to a full screen start menu to open programs, if they aren't pined on the task bar.
Windows 8 is the better OS: more secure, more stable, better performance, better cloud integration, etc. I'm almost always using desktop applications so it looks and works like 7. But I don't mouse to start menu items as it is faster using the keyboard and shortcuts so the menu format makes no difference to me.