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Dozens of Linux distros out there, MySQL is free unless you're selling a commercial app with it. No reason to run to Windoze because RedHat has changed their market priorities. Too many others to choose from.
Honestly, the only Windows app I'll touch is Terminal Server. Those cranky old PIs and 486's can run Windows with a remote boot and a Terminal Server session.
For a web based database you can't beat a Linux + MySQL combo. PostQ is fine I'm sure but I'm happy with the M. I have a site running with MySQL + PHP with 9 million records in it and it hasn't broken in two years. All the features of PQ and the others just don't apply for my needs, M does is as well or better (faster) with a smaller learning curve.
The biggest reason for me to stay with Linux (any Linux) besides the hundreds of obvious reasons is the remote console access. On the slowest machine with any OS I can get a secure shell into my servers and get the job done. No fuss. The whole idea of "My Computer, Documents, Pictures" and other nonsense has little meaning anymore.
And to clarify MySQL, it has become more liberaly licensed (more free) over the years. RedHat is less free and maybe better suited for those that want to pay for a support phone number. Take your pick of the many free distros.
"Friends don't let friends use Windows"
Apologies to the guy with the MS certificate. I just can't see running that stuff in the back room, but that's me.