Windows vs Linux web server

I'm working for a large client which uses exclusivly Windows environments.

My tasks are all based on their websites and one of those tasks is convincing them Linux is a better environment for hosting future and even existing websites.

What I don't know is how to convince them.

So, what is the best operating system for hosting php/drupal websites?

I apologize if this question does not fit this forum exactly but I would really value input on this.