The main differences between SM-SH56 and SM-SH51 cleats for cycling shoes are the release mechanism and the ease of disengagement. SM-SH56 cleats allow for multi-directional release, making it easier to unclip from the pedals compared to the single-direction release of SM-SH51 cleats. This makes SM-SH56 cleats more beginner-friendly and suitable for riders who may need to quickly disengage from the pedals in various directions.