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 Square Twist will create more pressure points on the mouthpiece which will make it stronger than the Round Twist and Smooth.