License For Textbooks — GNU FDL Or CC?

An anonymous reader writes 'I'm a college professor who is putting together an open-source textbook. I'm trying to decide between using the GNU Free Documentation License or the Creative Commons Attribution 3.0 United States License. I don't really understand the difference between these, though it seems with the Free Documentation License I need to include a copy of the license in my text. Which do you advise using?'