Advantage over Windows
No per seat/client licenses
Less Viruses Less Hacks Less Updating
Its Unix
Advantage over Unix
The SUPPORTED ability to run SOME ms software if you need to on a unix platform ( Exchange Server is set for later this year...late Summer I believe)
The propriatary hardware is cheaper than most other unix propriatary hardware ( Sun for example ) The cost of company support tends to be less as well. If your supporting it yourself and willing to build a cheap-tel box then forget this.
If neither of the above matter to you then hey do what you want, Just about any other arguement is opinion about liking this or that or maybe Aqua gui is your thing instead of Gnome or KDE...
I'm assuming your not talking labtop...If this is ever something you want to do DEFINATELY Mac OSX since you can dev, test and run light usage servers off one no prob. No other Unix will compare on this front.
Quicktime Server Streaming is freely available to Linux (and Windows and Solaris)
http://developer.apple.com/darwin/projects/streaming/
********** not directed at author but anyone stupid enough not to know ************
If you ever consider using OS 9 or lower for a Server you are being STUPID... while I will give the argument that the os will last if "you don't touch it" its not built to handle load...never was.