When comparing SDDM vs LightDM, the Slant community recommends LightDM for most people. In the question“What is the best Linux Display Manager?” LightDM is ranked 2nd while SDDM is ranked 7th. The most important reason people chose LightDM is:
Some greeters such as the Unity Greeter look absolutely beautiful on LightDM.
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Pros
Pro Supports both X11 and Wayland
Pro SDDM can start each DE
Unlike GDM which is stubborn in many ways, SDDM can start both Wayland and X11 sessions and any matter of valid session type, even if it's something obscure like Windowmaker.
Pro Recommended for Plasma 5 and LXQT
QML-based display manager. Successor to KDE4's KDM.
Pro Security
It doesn't support XDMCP.
Pro SDDM can provide full eyecandy
SDDM themes can include animated videos/gifs, background music/sounds, and any combination of the various QML animations.
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.
Cons
Con It does not support some essential funtionality, like XDMCP
Con Buggy and..
- Missing suspend/hibernate/sleep shutdown buttons
- Needs to much clicks to switch user
- Missing proper keyboard control
Con Slow
Con Difficult to customize without KDE
SDDM-KCM only works for KDE.
Con Does not support expired passwords
You can not force users to change passwords on login.
Con No Bluetooth support on the Loginscreen
Con Heavy
It's not very lightweight.
Con Customizing is hard
Customizing this login manager is hard since you need to know QML very well.
Con A bit bloated
It requires Qt which is a huge dependency compared to other login managers.
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.