Unix servers are far more stable - less reboots than Windows.
If you plan to use .NET and VB to develop your site, go for Windows hosting.
If you design with Unix in mind you can always host your site on a Windows server. Design with Windows in mind and you could have problems running on a Unix server.
Unix servers stand up to high loads far better than Windows servers.
My advice is to go for a Linux server, you'll be pleased you did, unless you're developing with .NET and VB, in which case it's Windows every time.