First difference: Mandriva 2007 DVD costs money to buy, whereas Fedora 6 can be made into a single DVD.
"Mandriva: Free 4 CD versions "
http://www.mandriva.com/en/download - Who has the patience to burn 4 CDs?
Second difference: In Fedore 6, you need to intentionally choose KDE/XFCE, in order to run the Fedora Core 6 smoothly. I am using XFCE now. GNOME is too slow!
Third differnce: Mandriva has central control center, liked by joe.pelayo, Jongi, myself and many others.
Fourth difference: Fedora core 6 takes somework to support MP3 files.