Medium Shank 7" sends a clear signal with more authority than the short.
The length of a bit's shank determines how much leverage transfers from the rider's hands to the horse. A combination of hands and horse will determine what shank length is best for your barrel pattern.
The three piece twisted wire mouthpiece is firmer.
All Merrill Bits have a three-piece dogbone mouth. These mouthpieces are designed to distribute pressure more evenly across the horse's mouth. Two-piece, or similar mouthpieces, apply considerable pressure to a single point and interfere with communication.