Depending on how you use it, OS X can feel a lot like Linux. If you're spending most of your time in Terminal and Emacs, usually ssh'ed into a server, then your technical answer will diverge pretty far from your "real" or metaphysical answer.
I work for multiple clients. After messing a few years with "I need a postgres for this one, a mysql for this one and sometimes, things just break on the live server because Mac OS X is almost linuxlike", I switched to a fully virtualized development stack and haven't looked back since. So the answer is not metaphysical, but true.
I can't comment on that. I don't develop anything that requires a GUI on the machine. I just ssh into the machine and do my thing there. All editing/browser viewing is done on the host.
This is my case - whether it's a Linux VM or on the cloud, the server OS is Linux.
For my desktop, OSX provides the best combination of spatial window management (you can drag-drop nearly anything - esp. text without wiping the clipboard buffer), terminal friendliness, and MS Office (yes, Excel is still better than alternatives). Lots of hidden gems and a sustainable indie dev market.
I don't play much more than the occasional Nethack, so my need for Windows is pretty limited.