Many different types of detectors are used for dual energy x-ray radiography. We developed an objective, technology independent, experimental method to compare the performance of these detectors. Our method uses a quality factor defined as the signal to noise ratio squared divided by the dose. We measure this quality factor by making images of a calibration phantom containing steps with known aluminum and plastic thickness. From the energy selective data, we compute an aluminum equivalent thickness image. The signal is the difference in the average value between two steps of the phantom with different aluminum thickness. The noise is the standard deviation of the computed values over the steps. The dose is the incident x-ray exposure. We applied this method to compare three energy selective detectors: (1) a single exposure, double screen, 'sandwich' storage phosphor detector, (2) an active storage phosphor detector using two exposures with different x-ray tube voltages with a light pulse erasing the front screen signal between x-ray exposures, and (3) a single exposure, double screen film detector. We found that the quality factor of the active detector is much larger than the single exposure film detector which in turn is substantially larger than the single exposure, storage phosphor detector.