When comparing XDM vs SDDM, the Slant community recommends XDM for most people. In the question“What is the best Linux Display Manager?” XDM is ranked 4th while SDDM is ranked 7th. The most important reason people chose XDM is:
It's a lot more lightweight than the most common GTK or QT based login managers.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Lightweight and fast
It's a lot more lightweight than the most common GTK or QT based login managers.
Pro Allows several seats
Pro Matured Software
XDM is not new, it has been in development since the 80s and is a very stable
reliable DM.
Pro Flexibility
Very customizable and portable, although with not as advanced effects.
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 Security
It doesn't support XDMCP.
Pro Recommended for Plasma 5 and LXQT
QML-based display manager. Successor to KDE4's KDM.
Pro SDDM can provide full eyecandy
SDDM themes can include animated videos/gifs, background music/sounds, and any combination of the various QML animations.
Cons
Con By deault you can't choose desktop environments
by default you can't choose your sessions/desktops, however there are plenty of tutorials on the web to add a selection.
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.