It depends on what you will be doing with the machine and what your work environment and collaborations are like.
My preference is to use OS X or linux (I use Linux Mint 17.1 64-bit with the Xfce desktop), but I need Windows for some applications and to ensure compatibility with collaborative files.
Although I would rather use Mac or Linux exclusively, I have been frustrated by the lack of software development and support for these platforms. By comparison, most of the software packages that I need to use are easier to install and use on Windows than on Mac or Linux. I have installed Win 8.1 on one of my machines, and by using the third-party classic start menu, it has the look and feel of Win 7, but it is faster.
I have also successfully run Windows and Linux on a new Mac Pro using VMs. I've compared VirtualBox, VMware Fusion, and Parallels. They all work, but VirtualBox does not have the graphics capability for graphics-intensive applications. VMware Fusion and Parallels are close, but Parallels works better for me, especially with graphics-intensive applications.
Whereas I really enjoy my Mac, if I were to do things over again, I think a smarter choice would have been to go with PCs instead of Macs. Almost all of the software that I use is available for Windows or Linux, and these OSs can be run natively on a PC.
BTW, my latest PC is a custom build from Puget Systems. It is the best and quietest PC I have ever owned. I can highly recommend this company.
Good luck with your decision.