Apple of course. Desktop pictures should be elegant, classy, colourful, all of which applies to the Snow Leopard pictures... but most of all they should be unobtrusive. With Windows 7 pics I would stress out before an hour would pass.
Hate to say it but Windows 7. They are more colorful and creative. OSX SL are kind of..... Blah and colorless. Really doesn't bring out the max potential of the Screen and Video Card.