While i'm not sure if XDM is lighter then SLIM, on archlinux forums there have been several threads about problems with slim recently.
Most of those problems are related to CK/polkit and systemd.
Those that did get slim to work, had to make changes in .xinitrc , put some programs in autostart folders etc.
XDM on the other hand needs very little changes to work with almost anything.
so my vote goes to XDM.