The Dee Ring is very similar in action to the Eggbutt, it will prevent pinching of the lips and gives a slightly more solid feel against the side of the face. It also prevents the bit being pulled through the mouth. The release is slower and some slight poll pressure is introduced.
The Mullen is a strong bit which creates direct pressure on the tongue. Used for a strong older horse and will also bring the head down.
The material used allows the bit to remain light and it encourages salivation. It is particularly well suited to horses that object to metal mouth pieces.