When comparing LXDM vs SDDM, the Slant community recommends SDDM for most people. In the question“What is the best Linux Display Manager?” SDDM is ranked 8th while LXDM is ranked 9th.
Ranked in these QuestionsQuestion Ranking
Pro Works with Nvidia
Works well with Nvidia.
Pro Only two dependencies
Only needs GTK and Xorg-Server.
Pro GTK2 and GTK3 versions
Pro Fast and fluent
Pro Works well on USB
A full install of arch on live USB works well on many computers with no text issues with LXDM.
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 SDDM can provide full eyecandy
SDDM themes can include animated videos/gifs, background music/sounds, and any combination of the various QML animations.
Pro Recommended for Plasma 5 and LXQT
QML-based display manager. Successor to KDE4's KDM.
It doesn't support XDMCP.
Con Requires Xorg-server
Currently LXDM does not support wayland.
Con Depends on GTK and its dependencies
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 It does not support some essential funtionality, like XDMCP
Con No Bluetooth support on the Loginscreen
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.