You can get even more fancy and have a boot option for both with specialisations!

specialisation.KDE.configuration = {
    services.xserver.displayManager.lightdm.enable = false;
    services.xserver.desktopManager.cinnamon.enable = false;

    services.xserver.displayManager.sddm.enable = true;
    services.xserver.desktopManager.plasma5.enable = true;
  };

But let’s not pretend NixOS is in any way beginner friendly.