I'm no expert i physics, but I did stay at Holiday Inn Select last night. Anyways breaking it down simply one method basically pushes the ball while the other strikes it. I would assume there would be a small amount of friction when the ball rest against the plunger when "pushing", so that may impact things. How ever when there is a gap, you lose that amount of energy until the ball is hit. Also I would assume there is a los of energy if the plunger is not EXACTLY centered on the ball.

So minimal you'd probably have to come up with a new unit of measure to even show a difference. Seriously, we are probably giving it more thought than a game designer would...