I hate to break it to you, but it's always so obvious when people are "trying" to sound complicated, it diminishes the validity of even your truest arguments.
We can't write your debate answers for you, and you should only consider these as a jumping off point for more research:
It's a fact that UNIX-based OSes like OS X are more secure because UNIX was designed with multiple users in mind, so security is layered throughout the entire OS. Windows OSes began with no security, and only along the way was some front-end security added.
Standardized assembly by the same company that makes the OS as the computer means efficient, high quality parts are being used, unlike some Windows-machine manufacturers that use sub par, even defective pieces in order to keep the overall price down.
You can run both operating systems on a Mac. You can't (legally) do so on a PC.
There is no real Windows equivalent of the iLife suite of programs that comes with every Mac. iMovie, iDVD, iPhoto, Garageband, etc are far beyond anything found on Windows. You can hunt around and try to find something created by a third party software company for free, but all the iLife apps are integrated to work together.
You can tell them that there is an enthusiastic, devoted developer following for Macs, so even third party apps are designed with to work with and even look like Mac's for a more seamless experience. There is a lot of software out there for Windows too, but much of it is unsupported or causes system conflicts, real trial-and-error.
The Windows Registry stores system wide settings in one place, and Windows gives administrator access to the first person who creates an account. This is dumb from a security point of view; Macs does not give root user access to anyone who doesn't first go through the process of enabling it, which is more trouble than it's worth to most users, so it remains safe and each user account remains stable so that a corruption in one account doesn't affect the others. Plus, junk program settings are thrown into the Windows registry along with every other, and moving an program can be a nightmare. It's one of the easiest ways to break an application.
Windows installers leave application files in so many places, you might never really be able to get rid of it. It puts shortcuts and system tray icons in for just about every piece of software whether it requires them or not. There's always an updater popping up somewhere, interrupting your work about ten times a day. OS X is clean, uncluttered and unobtrusive. I know I spend more time on my Mac than I ever did on my Windows box, partially because it's more efficient.
anyway, that should be a start.
