According to the below report AIX on POWER is still statistically the best OS for maximum uptime. I think that the comparison of the UNIX based operating systems are fair as these servers are likely to be doing similar jobs. The Windows results however are heavily skewed by the number of security patches that have had to be applied to the frontline servers. Is this a measure of the OS or a reflection on the position of the servers in the application stack/network layer?

http://windowsitpro.com/article/articleid/98475/windows-server-the-new-king-of-downtime.html

I would be really interested in a report that compared similar applications (e.g. ERP or database systems) running on AIX, HP-UX, Solaris and windows. 
Relative performance comparisons and benchmarking different systems is something that can be done in a finite amount of time with a lab full of kit. But uptime comparisons rely on surveys and user reports over an extended period of time, and never seem to be from well grouped application categories. If anyone has seen better stats that make more detailed comparison of OS uptime please let me know.

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