I’m not the one making blanket statements about one being cheaper than the other.
Cheaper is an absolute fact. I read a comparison last year, mail with 1U for 100 users. Exchange on a Dell ran about $17,000. An XServe ran about $6,000. Windows server OEM for 100 users will run you over $4,000, while OS X retail for unlimited users is $1,000. Microsoft kills you on CALs, and your company pays more as it gets bigger.
Ease of administration is not so concrete, but supported by the experience of every cross-platform admin I know, including one FReeper.
The question is whether these definite cost savings justify a switch from Windows to Mac.
I'm not saying any of this out of love for the brand. I would have laughed if you suggested OS 9 server for anything but a small Mac workgroup or standalone web server. But the UNIX-based OS X changed the game completely.