Well, linux servers and mac os x servers are classed in different categories. And linux powers the majority of web servers. OSX powers only a few percent (if that). Mac os x is not a form of Linux, its a form of Unix based upon Berkley systems distribution (freeBSD). Unix and Macosx are very different.
Differences between linux and unix:
Linux is licenced under GPL, unix is not.
Linux is not technicly owned by anyone, but unix is.
Linux kernal was writen before Unix so it was the original posix based OS.
etc.