When comparing XDM vs ly, the Slant community recommends XDM for most people. In the question“What is the best Linux Display Manager?” XDM is ranked 4th while ly is ranked 6th. 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 Minimal
Pro Simple in use
Pro Very quick and responsive
Pro CLI Based
It's so much lightweight because CLI Based.
Pro Supports both X11 and Wayland
Pro Easy to use
Includes a shell login and Basic Wayland support.
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 Doesn't pickup special characters
For example ß or æ.
Con Not very customizable
Con CLI-based
Very simple and minimalistic graphical interface.