In my opinion, there are positive and negative things about both.

Windows Vista:
Positive :: Stunning look, excellent graphics, neat effects. The new interface, including Flip3D (awesome equivalent to Alt+Tab) and SideBar, makes using windows much easier.
Negative :: A lot of software, such as many Adobe Products, do not correctly work on Vista. This is because Vista is a lame recreation of Unix.

Mac Leopard:
Positive :: Great new tools, such as Stacks,...

Stuart