Good previous comments in this thread.
I think it's important to be clear about the pricing issues for both XP and OS X.
For XP, if you are upgrading an existing PC, you have to buy a copy of XP, Home ($200) or Pro ($300), for each machine. Microsoft makes sure you follow the rules on this. The upside is that I think MS holds up their end of the deal by delivering a solid Windows OS.
For Mac OS X, the retail price is $129, but I'm seeing it for $99 more and more. Even though Apple asks that you buy a copy for each Mac in your household, the truth is you can buy one copy and use it on your laptop and your desktop without Apple busting your chops. In general, I feel that Apple stays out of your business better than Microsoft.
If you buy a new Mac or a new PC, you get the OS as part of the deal, so it's a wash there in terms of OS cost.
As for pure value, Mac OS X is a better deal. It includes the OS, Apache Web server, iPhoto, iMovie, iTunes, and iTools, which is an under rated Apple value-added benefit.
If you're currently running Mac OS 9, I recommend that you partition your drive and load Mac OS X on one partition and your old OS 9 environment on the other. OS X is much more stable than OS 9, and I wouldn't want to go back to 9 anymore than I would Win 98.
Life is just too short ...