Electrostatic and piezoelectric electromechanical coupling are employed in miniature devices to produce ultrasonic waves or generate power. It has been pointed out in the technical literature that in principle electrostatic devices can be designed to have an electromechanical coupling factor of nearly 100%, while it is thought that the upper limit for piezoelectric devices is significantly smaller. We have recently developed a closed-form model of a thin-film piezoelectric device to predict the performance of membrane piezoelectric energy converters. The model was used to identify several key design and process parameters that have a substantial effect on electromechanical coupling. This model is general enough to allow a comparison of the two technologies, electrostatics and piezoelectrics, at a lower level of detail. In this paper, the model is used to compare the components of the electromechanical coupling factor; capacitance, stiffness, and actuation force, for the two energy conversion technologies. The comparison shows that the capacitance and actuation force coefficient are drastically different for the two technologies, and are controlled by fundamental material properties and device geometries. Consequences of the differences for the design of ultrasonic transducers and power generation devices are discussed.