The Medium Ring Gag is stronger than the usual Loose Ring and Eggbutt as the ropes going through the rings create leverage which will exert more pressure through the mouthpiece whilst at the same time introducing more poll pressure.
The Waterford bit has many smooth flexible bumps or bubbles, which spreads the pressure over many points making it a light mouthpiece. The greater flexibility discourages a horse from leaning on it.
The Waterford bit is a good choice for a horse that leans, it also promotes mouthing and salivation. The Waterford is usually worn ¼" - ½" longer than your traditional mouthpiece in order to curl around the lips and maximise the effect.