Both Linux/Unix and Windows can handle high traffic websites and add-ons such as chat rooms, email and website statistics with ease. The biggest differences are price and software compatibility. Because Windows must be purchased and licenses renewed continuously, Windows hosting plans are usually more expensive than Unix-based plans. Also Unix-based plans often come with free email, databases (MySQL), statistic, and community programs. While Windows plans come with Microsoft software (MS SQL, for example) that must be purchased by the hosting company.
In low-stress conditions Windows and UNIX servers have very similar performances but under high load UNIX is very superior to Windows.
UNIX handles high server loads than Windows; UNIX systems rarely require a reboot, Windows constantly need them. UNIX can run stable much time than a Windows server and UNIX up-time is bigger.
If you design and program a website under a UNIX server you can easily host it in a windows server, but the reverse is not always possible, Avoiding windows languages (ASP, ASP.NET, etc) and databases (Access and MS SQL) is the better choice.
Any server hosting a website require a operating system and licenses, Windows 2003 and applications like SQL Server costs a lot of money, on the other hand, UNIX, usually Linux, is free to download, use and operate. This make the difference between the price, UNIX hosting will always be cheaper than a windows one.
Unix hosting is more stable, has better performances, is more compatible and cheaper than a windows hosting plan. I always recommend UNIX hosting. You will only need Windows hosting if you’re going to develop in ASP, .NET or VB.