Crank or mid-motor

The alternative to a hub motor is a crank or mid-motor.

The main advantage of a crank motor is that it handles hills better than a hub motor because it drives the front crank mechanism and hence the chain.  In doing so, it takes advantage of the bikes gear system which dramatically improves the motor's efficiency.

Pros

1. The crank motor is more powerful than the hub motors and tends to be better on hills, where it is equivalent to a much larger hub motor
2. It tends to be faster due to it's operation through the bike's gears

Cons

1. It is slightly more obtrusive and perhaps less "sexy" than the hub motor, but some people even prefer its looks.
2. More expensive than the hub motors