Day to day tasks, I'm pretty equally efficient in any OS, but OSX's significant "lifetime of ownership" time savings for me has been system maintenance. I've never
needed to reformat and reinstall a Mac, and when I've voluntarily done it (hackintoshing), restoring a complete system from Time Machine backups has been absolutely painless.
Major OSX updates have never been a problem either (although I've read accounts of errors happening), I've carried one Macbook from 10.4 to 10.6 without ever reinstalling. I've been burned a couple times trying to do distribution upgrades with Ubuntu, I've never had a Windows install that didn't need an occasional refresh for some reason, and I wouldn't dare upgrade from XP->Vista->7. I've not owned Win 7 long enough yet to determine if that will still be the case, but Windows' history has me skeptical.
Just my experience. The Mac price difference has paid for itself many times over for systems I depend on and family I support.