Yes, Mint uses Slick-greeter. I personally do not...
Cinnamon Desktop comes with numlockx. Setting it to autostart is a simple addition. You can do this with either lightdm or slick-greeter - which slick-greeter really is lightdm, slightly forked.
But I prefer LightDM because it allows me to tweak things, change things more easily in just one location. And while the Slick-Greeter GUI comes with a couple mroe switches to flip, it lacks a few that Lightdm-gtk-greeter-settings GUI has... Plus, I can just go into the lightdm.conf and change at will, anyway.
It really comes down to personal preference, how much configuring you want to do and what you are familiar with.
For someone who does not care about customizing or configuring and only uses default- then gdm is as good as any.
If they prefer using what thy are used to on Mint, then Slick-greeter is the way to go.
If they are used to how they did it on XFCE, then LightDm is the way to go.