Using the second pad, at least when shooting a handgun, causes your wrist to turn inward. Try it using the index finger on your opposite hand as your "trigger" and you will notice how it your wrist wants to "break" inward.
Using just the tip of your finger helps to keep your wrist straighter in my experience. As a result, not matter what I'm shooting, I use the tip.
"He who laughs last, laughs dead." Homer Simpson