The two most important atmospheric transmission bands in the infrared occur at the midwave band of 3-5 μm and the longwave band of 8-12 μm respectively. For a given infrared detector a common question asked is, of the two, which, if any, gives the better performance? A question not without controversy, in this study an analysis designed to assess the relative merits of infrared detectors operating in either of these spectral bands based on the recently defined thermal figure of merit known as the detected thermal contrast is undertaken. Under ideal limiting conditions, by considering a range of targets whose spectral emissivities vary as a function of both wavelength and temperature, we impugn a number of results previously reported in the literature regarding the performance of a detector made using detected thermal contrast as the thermal figure of merit. For the two broad types of detectors considered it is found the midwave band for both thermal and quantum detectors gives better performance for a range of different target types.