Do you really need a motion sensor? Any well designed home/et al should have a switch at both ends of stairs for an outlet that lights them.
Then again, unless the downstairs is living area, a switch at the top would work as you'd be coming back up soon enough and a decent LED setup won't burn out any year soon and uses little power to worry about having it on a bit longer.
I suppose I have an aversion towards motion sensors as they never seem to last more than ~ 6 years, though most of my experience using them is outdoors where sunlight might take its toll.
Maybe that's just my excuse for avoiding additional wires and expense.
