Unix boxes (least in the early days) were renowned for withstanding heavy traffic that would take a Win box down.
I use php because all of the scripts (forum, blogs, tracking) ...ect..ect I've used in the past were written in php - this is going back to 2001. So it was a natural progression to learn php. In addition I've always hosted on Unix with Apache, Red Hat or similar.
I suppose each camp will always appose the other much like Holden enthusiasts don't care for Fords!