When comparing MDM vs GDM, the Slant community recommends GDM for most people. In the question“What is the best Linux Display Manager?” GDM is ranked 8th while MDM is ranked 13th. The most important reason people chose GDM is:
GDM is dull, but it just works, and it is highly stable. It's easy to switch between environments, and it integrates really well with Fedora or other Gnome Distros.
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Pros
Pro Easily user-configurable
Pro Many available themes
Hundreds of themes.
Pro Excellent documentation, easy-to-find/understand
Pro Mature software
Pro It just works
GDM is dull, but it just works, and it is highly stable. It's easy to switch between environments, and it integrates really well with Fedora or other Gnome Distros.
Pro Can run Wayland sessions
GDM can start Wayland sessions, which is the default for recent GNOME versions.
Pro Always signs the GNOME keyring
Works with any desktop. SOome display managers such as SDDM don't open the GNOME keyring at login.
Pro Not so difficult to customize
You just have to know which files to edit, and you can do quite a lot.
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 Tied to Gnome
You basically have to use Gnome or one of its forks to use it properly.
Con Really hard to customize
You'll have to recompile Gnome's resource files in order just to change the login background.
Con Depends on GTK and its dependencies
Con Ugly UI
It is very simple, and you can't change the layout.
Con Bloated
As with anything Gnome, there's a level of inelegance to be expected when it comes to absolute performance. It's supposed to be a login manager, not something bespoke.