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.
Dog bone or dog chain mouthpiece is mild and less offensive.
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.