When comparing Qingy vs XDM, the Slant community recommends XDM for most people. In the question“What is the best Linux Display Manager?” XDM is ranked 4th while Qingy is ranked 16th. 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 Getty replacement
It is a getty replacement so it will not only replace your X11 login manager but also your getty.
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.
Cons
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.
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.