I asked the same question for Freya but in Loki the situation is different, none of the two dialogues can be triggered with those commands.
gnome-session-quit --power-off used to trigger this dialogue:
while the default dialogue was triggered by the command
dbus-send --session --print-reply --dest=com.canonical.Unity /org/gnome/SessionManager/EndSessionDialog org.gnome.SessionManager.EndSessionDialog.Open uint32:2 uint32:0 uint32:0 array:objpath:.
Asking on the g+ group here, the command
dbus-send --system --print-reply --dest=/org/freedesktop/login1 org.freedesktop.login1.power_off was mentioned, but it doesn't work.
What is the command behind the default Loki Shutdown dialogue? I prefer to use a shortcut for that, especially that the shutdown laptop button triggers no dialogue but suspends directly.