I said it's situational. Unlike you, I'm not dogmatic. I believe in the best tool for the job and circumstances.
Doing a lot of content creation? OS X all the way.
Do I have the money and want to set up a small business office requiring a wide range of desktop and server applications? Probably OS X again.
Actually, anytime you're looking for low power consumption and brain-dead user friendly with a large set of available professional-level applications, it has to be OS X.
Setting up a couple hundred systems for users to do basic office work and intranet apps? I'd go with Linux.
A bunch of public computers at the library? Linux, unless you're in a rich neighborhood that can afford OS X.
Looking for five nines? I'd probably go AIX or Solaris, although Linux is at least worth a look now.
Do I need low-cost but reasonably reliable router/firewall/DNS/NAT/SMTP/POP/FTP/LDAP...? Linux, of course.
Do I want a quick, cheap, buy-install-run high-performance computing cluster without having to be an HPC expert? Apple XServes with OS X and XGrid.
Setting up a very high-volume, highly transactional, high-reliability database server and can't have a cluster? Probably Solaris.
Setting up a fairly custom job and need to be able to tweak anything in the OS, but still need a wide userbase of support? Linux is the only way to go.
Windows wouldn't be a choice for me in any of those unless there was a specific overriding reason that required me to use it.
The biggest reason for Solaris above is that it has been rigorously tested on Sun hardware, which isn't something you can usually say for Linux, and the serious combination hardware/software support from the one original vendor. With Microsoft or Linux you get the software vendor telling you it's a hardware problem and the hardware vendor telling you it's a software problem. Apple has this advantage too, but IMHO they're not mature enough for high-end, high-availability vs. the other 'nixes (wait until 10.5 and everything moved to Intel).
Otherwise, the choice of Solaris vs. Linux is usually what the person is comfortable with.
Solaris is free for use, why can't you use it in the library? Why can't you EVER speak out against the fanatical leftist Stallman, and work to only use his products when absolutely no other option exists? You did say he was a radical leftist, right? You prefer using software from radical leftists, when other free options exist?