Ready for another "schoolin"?
You know we were talking about user experience, so you switched from Linux to Max. But since you now say all modern *NIX...ok. Take this bet then. Windows is more useable by the average person (from install to daily ops). We give a copy of linux to any 5 typical users and a copy of Windows to the same users. We ask them to do several common tasks. I bet the windows platform has them completed quicker on average than the linux platform. Is that a bet or were you just talking out your rear-end again.
My *NIX comments were about passwords, since they all use the same password system. Usability varies greatly between *NIX systems. But in your case the outcome would depend on whether these are previous Windows users, what distro you use, and whether you choose to use the Windows-like GUIs in Linux. Yes, they look just about like Windows. You could always adjust your competition so you win.
The outcome against Mac just wouldn't be fair. Install an application? Let's take MS Office. I can either go through that long install process with Windows, or I can just drop it from the DVD onto my hard drive with OS X.