I agree with what Reel Time said.
When you are on plane your boat will more or less be 'leveled out' (for lack of a better term). When you are getting on plane, the bow of the boat rises, the stern of the boat 'squats'. As you pick up speed the front begins to lower and the back begins to lift up. The boat is actually rising on the water. When you are on plane and trim the motor up, that causes the bow to rise even further, thus putting less hull in the water. Therefore, less friction between the hull/water and a faster more efficient ride.
As far as the bet goes: You are correct in that you were on plane. He is correct in that the bow would be raised a little further, thus reducing the amount of spray that would come over the gunnel.
Therefore, it depends on what you were betting on as to who is the winner. Perhaps, both winners?