When comparing CDM vs SDDM, the Slant community recommends CDM for most people. In the question“What is the best Linux Display Manager?” CDM is ranked 3rd while SDDM is ranked 8th.
Ranked in these QuestionsQuestion Ranking
Pro Has almost no dependencies
Pro Supports virtually any DE/WM
Pro Very lightweight
takes up near no system recources.
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 Bad availability in distros
There is no CDM in distros repositories (only in AUR on arch)
However you always can compile it from source.
Con Curses-like interface
Very simple and minimalistic graphical interface.
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.