When comparing CDM vs XDM, the Slant community recommends CDM for most people. In the question“What is the best Linux Display Manager?” CDM is ranked 3rd while XDM is ranked 4th.
Ranked in these QuestionsQuestion Ranking
Pro Has almost no dependencies
Pro Supports virtually any DE/WM
Pro Very lightweight
takes up near no system recources.
Pro Lightweight and fast
It's a lot more lightweight than the most common GTK or QT based login managers.
Pro Matured Software
XDM is not new, it has been in development since the 80s and is a very stable
Pro Allows several seats
Very customizable and portable, although with not as advanced effects.
Con Bad availability in distros
There is no CDM in distros repositories (only in AUR on arch)
However you always can compile it from source.
Con Curses-like interface
Very simple and minimalistic graphical interface.
Con By deault you can't choose desktop environments
by default you can't choose your sessions/desktops, however there are plenty of tutorials on the web to add a selection.