I have spent a fair amount of time working with PHP in both IIS and Apache server (Windows) configurations. I have seen suggested a couple of times that Apache is preferred over IIS.
I had been under the impression that one of the primary reasons for Apache over IIS was that performance under CGI through IIS was significantly slower. However, the support through Fast CGI seems to have fixed much of that issue.
Another issue is certainly licensing costs, but assuming that isn’t an issue, what are the other differences, assuming a Windows-based platform?
You’re right, IIS FastCGI nullifies any arguments about PHP on IIS being slower than Apache. Assuming a Windows Platform, leveraging IIS gives you all of the benefits that come with using IIS like: a brain-dead simple path to clustering; leveraging the Web Platform Installer to get a jumpstart on running common PHP apps like Drupal; and the ability to combine technologies like PHP and ASP.NET in the same website.