What are the key differences between various pedal-assist electric mountain bike systems, and how do these differences impact the overall riding experience, particularly in terms of power delivery, battery life, and weight distribution? Are there any specific benefits or drawbacks to consider when choosing between a mid-drive motor versus a hub-drive motor, and how do these choices affect the bikes handling and maneuverability on technical trails? Additionally, what role do factors like gear ratios, suspension design, and tire choice play in optimizing the performance of a pedal-assist electric mountain bike, and are there any emerging trends or technologies that are likely to shape the future of this type of bike?