I hadn't used either until one night a few weeks ago, my wife had gone to bed early and I was watching Ghostbusters on dvd... in that case I used Night mode to reduce the chance of waking her up...

I'd say that if you're watching something and don't want to disturb someone, hitting a "night mode" button would make much more sense because it could be turned on or off quickly (like right at the start of a loud passage)