There was a discussion on this board that touched on this a little while ago, I think the starting message was entitled "Linux the Amiga of the 00's". When our company needed to make these choices we set up trial servers and then ran load, stability and timing tests to see how each one performed. Apache is free, and you can get evaluation copies of NT and IIS if you subscribe to the complete MSDN package. I personally think Apache running under Linux is the way to go, but others will disagree