Well, if you're taking suggestions for different OS's as well, then I would suggest comparing BSD with Linux. Maybe test out Solaris, as there seems to be some questions about that. Maybe you could also check out one of the Windows Vista betas and compare it with that of xp and/ or 2000. Hell, you could even check out some hobby OS's like Menuet, BeoS, ReactOS, AtheOS, or any of the ones here at
http://en.wikipedia.org/wiki/List_of_operating_systems.
Maybe not all of those, but if you've got the time you could compile a list of operating system comparisons from an experienced computer user such as yourself

.