Are you asking whether you should use OS X or Windows, or whether you should use Apple-brand hardware or Dell/HP/etc./home-built generic PC hardware?
Because if you're really comparing OS X to Windows, then lets be honest, you can run OS X on PC hardware fairly easily, although it's even easier to run Windows on Mac hardware.
But if you're comparing Apple hardware to PC hardware, I mean, if you're really a designer, then you shouldn't need me to explain the difference. It should be quite obvious. As a designer, I'd think you of all people would appreciate the primary advantage of Apple hardware: its design. If you think design doesn't make any difference, then why the heck are you a designer?!
For example: the Apple-brand trackpads (both the ones built into their laptops and the ones they have for desktop use) work phenomenally well. They work best in conjunction with OS X with the native drivers, but Apple's written fairly decent drivers for them for Windows now.
The cursor tracking in the Mac operating system has always been smoother, since 1984 until now. I've always found Windows to be kind of jerky and have noticeably worse input handling for mice and (especially) trackpads. Apple's laptops have, by far, the best trackpads of any brand, and once you get used to how you can use gestures for many tasks on Apple's trackpads you'll never, ever want to go back. Especially as a designer, where precise control of the cursor is very important, I think you would appreciate this.
Also I really like the stability and performance of OS X. Being based on unix it has a number of advantages. Windows as an operating system is just not as solid, performance wise, except when it comes to games. Macs simply handle multi-tasking and running tons of programs at once a lot better than Windows.
Another thing is licensing issues. If you have a Mac and you want to upgrade your computer at some point, or if your computer dies, you can just take the hard drive out of the one computer and pop it into the other computer. Or if you're using a Retina MacBook Pro or Air that has the built-in SSD, you can restore from a backup very easily without worrying about your motherboard drivers being incompatible and Windows just blue-screening on you. Or you run into the thing where your Windows was licensed just to the OEM maker of your last computer, and won't work on the new system without buying a new Windows license. Even then, good luck getting your motherboard drivers from the old system to work on the new one! This can really be a nightmare!
I deal with Mac and Windows both at work. I'm an IT professional, a designer, and a programmer. All of the professional web developers I know do everything on Macs. There are a lot of other reasons for it.
Of course there is a lot of bias against Macs out there, and you will run into lots of ignorant people who simply don't understand. They hate Apple for reasons completely unrelated to reality. Personally I think it's because they are mentally incapable of perceiving the difference.
Also there is a strong argument for just going with what you're most familiar with. If Windows is what you've always known and loved and used, then OK. It's not THAT bad. There are some things I like about Windows better, but nothing worth mentioning. If you combine that with being mentally incapable of perceiving what the things are about Apple's hardware and software that are better, then well, you have a result where the person is better off sticking with Windows.
Remember though that VirtualBox is free and with the power level of today's computers, you can run Windows XP or 7 or 8 or whatever right alongside your Mac system and it works flawlessly. It's not emulation, it's a virtual machine running native on your Mac's CPU. I'm pretty much always running Mac and Windows at the same time on my MacBook Pro for web development purposes and testing.