Comment 2 for bug 136433

They are different, because the disapper of those buttons was after upgrading the package "powermanagement-interface" from 0.3.15ubuntu1 to 0.3.16
The button in the shutdown-menu, and the menu-item in the powermanagement systray-icon, are gone.

I could not downgrade the version, because there is only this new version left in the repository.
But after removing this package (powermanagement-interface), the button reappears (after reboot).