Ok Google Now (touchless controls on the settings page of the phone) works when the screen is off. The "OK Google" support in the Google search app (and in Google Now) requires the screen to be on.

That's the main difference. Touchless controls also supports "OK Google Now, read notifications" (or "What's up?") to read any unread notifications in the notification shade. I don't think that the Google search app supports that.

 

Welcome to Android Forums.

You need to experiment with it.

On the Droid Maxx I recorded "OK Droid Now" and it responded to "OK Droid" ... like "OK Droid weather".

I had to try recording it a bout 25 times until it recognized it.

The next generation touchless control is much more forgiving.

... Thom