Some bikes, e.g. Busettii Vortex use an external motor, while others (by the same manufacturer) use hub motors. Busettii claims that external motors have higher efficiency (http://www.biyadii.com/), yet their top-of-the-line Big 50 model uses a hub motor. Other Web sites say that hub motors are more efficient.
I'm confused. Do external motors really produce more torque using less electricity? If so, how? If the issue is sub-optimal RPM in hub motors, then why would moving the motor outside the hub help with that? I am looking for some no-nonsense technical explanation of the pros and cons of each design.