When comparing SDDM vs GDM, the Slant community recommends SDDM for most people. In the question“What is the best Linux Display Manager?” SDDM is ranked 4th while GDM is ranked 5th.
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 It just works
GDM is dull, but it just works, and it is highly stable. It's easy to switch between environments, and it integrates really well with Fedora or other Gnome Distros.
Pro Always signs the GNOME keyring
Works with any desktop. Some display managers such as SDDM don't open the GNOME keyring at login.
Pro Not so difficult to customize
You just have to know which files to edit and you can do quite a lot.
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 Tied to Gnome
You basically have to use Gnome or one of its forks to use it properly.
Con Depends on GTK and its dependencies
Con Really hard to customize
You'll have to recompile Gnome's resource files in order just to change the login background.