It's always a personal choice. Personally, I say:

A.) Windows 7. You cannot purchase OS X for your own computer; you must buy a Mac, which costs more than most other computers, spec-for-spec. Upgrading, Snow Leopard is cheaper, but not compatible with hardware that's more than 3 years old; those users must now buy a new computer. XP users from 2001 do not.

B.) Tie. (Depends on the hardware configuration.) Most pre-built units in the same price range are about equal, but you can build your own Windows machine with better secs for less.

C.) Windows 7. Snow Leopard doesn't let you choose WMP, but Windows 7 does let you choose WMP, which I prefer.

D.) Tie. I like Windows 7's approach to task switching, but performance-wise they are about equal.

E.) Windows 7. Snow Leopard is cleaner and easier for new users to use, but I prefer Windows 7, with customizations. OSX's interface has been largely static, minus the dock.

F.) Snow Leopard. I have access to Microsoft support options that most don't, so my issues have been quickly resolved. But from what I've experienced from the average user's perspective, there is a lot that could be improved upon.

John