The main difference between each system is the availability of reports. Because the comprehensive unit is installed in your vehicle, that means additional reports are available such as working times, driving times, idling. This is ideal for companies who want to reduce their administration or comply with laws such as the working time directive. However, if you just want your vehicles location, then the portable unit is a cheaper option.
The iPhone is the cheapest option. However, it tends to be used in a different way - to send bursts of locations say when at a customers rather than continually send its location. This is due to the limitation of the iPhone which means that third party applications cannot run in the background.