you can switch between KDM and GDM by installing system-switch-displaymanager, which you can run from the command line to select your display manager of choice. In both you can select if you want to log into Gnome or KDE. personally I think KDM is a little more easy to use and configure.
"$ su - bofh"
OS: Mac OSX Snow Leopard (x64 by default), W7_Enterprise_X64, F12_x64_KDE
Hardware: late 2009 Macbook 13", MSI Wind 10"
Browser: Opera 10.10, Safari 4
Registered linux user #459910