Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Only 15 years ago, Linus Torvalds was annoyed enough about the expense of the just-commercialized Unix workstations and systems on the market that he started to create his own operating system, one that had a look and feel very much like Unix. Back then, proprietary operating systems dominated the midrange of the server market, mainframes ruled the high end, and there was no such thing as an entry server, but rather a NetWare file system for file sharing and network printing. How the world has changed. Proprietary operating systems still have their niches--and they will for a long time to come--and a formerly ascendant Unix has been pushed out of the entry server space by Linux and Windows.
Make no mistake. Unix systems, mainframes, and even AS/400s (now known as the System i5 but rarely called that) are still dominant in the data centers of the largest companies in the world. They do a lot of the heavy lifting in terms of batch jobs and transaction processing. But, then again, at many companies, Windows is used for new applications or has even supplanted Unix, and at others, Linux, because it is like Unix excepting its relatively high price, has ousted Unix. Unix platforms have been under assault for a number of years, but Unix vendors are starting to fight back, each in their own ways.
Doing any sort of comparison right now is a bit difficult, because a lot of the core server platforms are in flux right now. The changes underway only play into the hands of the Windows platform as X64 and Itanium servers are making their way to market offering more bang for the buck. Of course, what helps Windows on a server hardware platform usually also helps Linux, since they run on the same iron. But, because Hewlett-Packard supports HP-UX on the same Itanium iron as it supports Windows and Linux, because IBM supports AIX on the same Power iron that it supports Linux and OS/400 (now called i5/OS), and because Sun Microsystems is shipping Opteron machines that support Windows, Linux, and Solaris, the playing field for operating systems is, in some cases at least, more level that it has been in years. Unix has a fighting chance on many platforms, but then again, so does Linux.