When comparing SDDM vs Qingy, the Slant community recommends SDDM for most people. In the question“What is the best Linux Display Manager?” SDDM is ranked 4th while Qingy is ranked 16th.
Ranked in these QuestionsQuestion Ranking
Pro Supports both X11 and Wayland
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 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.
It doesn't support XDMCP.
Pro Getty replacement
It is a getty replacement so it will not only replace your X11 login manager but also your getty.
Con No Bluetooth support on the Loginscreen
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 Ancient fossil dredged up from nearly a decade ago
Qingy doesn't appear to have updated since 2010. That's a long time in Internet years.
Con Not a display manager
Qingy is not a display manager - it is a getty replacement.