When comparing GDM vs LightDM, the Slant community recommends LightDM for most people. In the question“What is the best Linux Display Manager? ” LightDM is ranked 1st while GDM is ranked 2nd. The most important reason people chose LightDM is:
Ranked in these QuestionsQuestion Ranking
Pro Always signs the GNOME keyring
Works with any desktop. Some 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.
Pro Incredibly lightweight
Pro Beautiful interface
Some greeters such as the Unity Greeter look absolutely beautiful on LightDM.
Con Tied to Gnome
You basically have to use Gnome or one of its forks to use it properly.
Con Depends on GTK and its dependencies
Con Really hard to customize
You'll have to recompile Gnome's resource files in order just to change the login background.
Con Not really lightweight
How lightweight it is depends on the used greeter, but they all require much more dependencies than other lightweight greeters like XDM or SLiM.