A solenoid makes more efficient wiring. Motors are used in situations where continuous movement is required, rather that just an on-off response. Stepper motors are often used, as they can accurately control movement. The fuel injectors are the main actuators of the vehicular electronic system. It is good to focus on this actuator as students can identify with the need for fuel in the system.
Fuel injectors can be very complex, and yet their theory can be explained very easily.
A longer duration produces more quantity, of course. They are simply a solenoid, operating from a coil of wire that moves the pintle valve. A pressurized system behind the valve allows the spray to occur. The literature is alive with acronyms to describe fuel injection systems as being unique to the manufacturer. Again, things are a bit more complex than they need to be for student understanding.