When comparing MDM vs XDM, the Slant community recommends XDM for most people. In the question“What is the best Linux Display Manager?” XDM is ranked 4th while MDM is ranked 13th. The most important reason people chose XDM is:
It's a lot more lightweight than the most common GTK or QT based login managers.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Easily user-configurable
Pro Many available themes
Hundreds of themes.
Pro Excellent documentation, easy-to-find/understand
Pro Mature software
Pro Lightweight and fast
It's a lot more lightweight than the most common GTK or QT based login managers.
Pro Allows several seats
Pro Matured Software
XDM is not new, it has been in development since the 80s and is a very stable
reliable DM.
Pro Flexibility
Very customizable and portable, although with not as advanced effects.
Cons
Con Might not work well with distributions that are based on more recent packages
This is a display manager forked from GDM2 by the Linux Mint developers. So the display manager is developed alongside Linux Mint, and since Linux Mint is based on the latest Ubuntu LTS it might not work well with distributions that are based on more recent packages.
Con Depends on GTK and its dependencies
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.