Remember, I'm talking about IIS v6.0, included in Windows Server 2003. This may or not apply to any other IIS version.

Why is IIS better?
Ease of administration, modular use, more compatitility, tighter security, faster thoughput, ability to handle more simultainious connections, more customizable, full GUI management - can be controled through web interface as well, and last but not least, it can do everything that Apache can do and MORE!

I can go on more, but only if you are interested.