BellHiller makes the heli more responsive. It takes about 5 minutes to install and costs about $25. I wouldnt recommend it to someone learning because it makes the heli react so quickly to input. Usually the problem people have when learning on a heli is that they overcorrect. It takes a while to learn to only move the sticks 1/8 of an inch as compared to an inch. Overcorrecting one way will usually cause you to overcorrect the opposite way to regain control. Which very seldom works.
Id have to say stick with the stock head till you want faster response.