I like a heavy head for the penetration advantage, and pick a stiff enough shaft to accommodate the heavier point weight. A heavy head, providing the arrow is stiff enough, helps stabilize the arrow and leads to good arrow flight, and, since momentum is a vector force, meaning it "pushes in the direction it is headed", the lighter shaft tends to follow where the heavy head is going, and the force of the arrow is concentrated in the direct the head is travelling - imagine throwing a hammer.