When comparing LightDM vs GDM, the Slant community recommends LightDM for most people. In the question“What is the best Linux Display Manager?” LightDM is ranked 2nd while GDM is ranked 8th. The most important reason people chose LightDM is:
Some greeters such as the Unity Greeter look absolutely beautiful on LightDM.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Beautiful interface
Some greeters such as the Unity Greeter look absolutely beautiful on LightDM.
Pro Best balance
Best interoperability, best balance between functionality and bloat, simplicity and candy, not distribution or GUI-tied.
Pro Cross-desktop
LightDM is not tied to any distro or desktop enviornment. You can use it on literally any Linux GUI.
Pro Good for DEs without a display manager
If your Linux desktop does not offer a display manager, you should use LightDM.
Pro Theme variety
There are a wide variety of themes available for LightDM, from console-like UIs to ones that utilize webkit2 to create flashy and dynamic login interfaces.
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 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.
Con nVidia driver and kernel woes
It's been on the decline lately where it's only stable under a very specific mix of kernel and nVidia drivers.
Con High RAM usage
It uses more RAM than other light (xdm,slim) display managers which results in more overhead.
Con Poor/missing documentation
Con Hard to configure
Lighted.conf doesn't even work.
Con Autologin never worked
After 4 years of using LightDM, I never managed to make it autologin my user in the system and there are always problems with the graphics driver. Crashes too often and requires manual restarting which is dull. Not to mention the developers update it once in a leap-year.
Con Feature creep
Most people don't need (or even know) all features of LightDM.
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.