As user of Linux, Mac and PC I can give you an unbiased opinion of all of them:
None are better than the other, except maybe in some users opinions. Each has their pros and cons. I use the right tool for the job at hand. I would be cautious of any user, even if technically inclined, who makes blanket statements about one being much better than the other.
Lets face it. There are several million installs out there of Windows Server OS's. They are there for a reason, and it is not because those purchasing they systems don't know what they are doing. There is certainly nothing to be "cautious" about.
What I would be concerned with is who is going to maintain it for you. If you know Linux, then by all means use it to store your media files. If you don't, then an OS you are more familiar with would be a better choice. Windows Server OS's are very similar to the Desktop Systems so you would probably be able to maintain them yourself. Either way MC would have to be running on a Windows PC, so it is really a moot point.
Personally, I wouldn't run MC on a server OS unless there was some good reason to. You won't gain anything just because it is a server OS except maybe complexity. If you aren't sure of the reasons, stick to a desktop OS...
If you could give a little more information about what you trying to accomplish, I may be able to provide you with better guidance.