The best thing you can do is go out and use them. Otherwise you're asking "Which is best, chocolate or strawberry?" It's very much a matter of taste (no pun intended.) Some people like one, some like the other, some like both.
The deciding factor in some instances is going to be what you want or need to do with the system. Need it for work? The corporate IT department may have policies or preferences you need to adhere to, or you may have a program that runs in one but not the other. You may have a program, work aside, that is only available on one but not the other, or that runs better on one of them. You'll have a wider variety of hardware choices with Windows, of course, so that may be a deciding factor - but that has nothing to do with the operating system itself.
Are you building your own system? In that case, you're going with Windows (or Linux.) Yes, you can buy a "hackintosh," but again, your choice of compatible hardware there is more limited.
Personally, I'm not a fan of Windows 8.1 - I've run 8 since the developer preview and haven't liked it in the least, so for me the choice would really be Windows 7 or OS X. And since I've got a large library of programs on the Windows platform, I'd go with Windows in most instances. But that's just me.
source- yahoo answers