This paper reports a comparative investigation of solidsolid phase change material (SS-PCM) and solid-liquid phase change material (SL-PCM) for cooling of FGPA devices used in avionics applications. The study aims to analyze the thermal performance of a pin-finned heat sink packed with these two PCMs. A Polyethylene Glycolbased Form-stable SS-PCM and a commercial organicbased SL-PCM named A24 are chosen for this study. The two PCMs are selected to have nearly the same thermophysical properties and operate at a similar temperature range within the scope of the given application. The characterization is carried out for both PCMs for their thermal properties, such as latent heat, phase transition characteristics, specific heat, thermal conductivity, and thermal degradation. A numerical model is developed for the transient heat transfer study of the two PCMs under the same heat input rates and charging-discharging time. Parameters such as the mass of the PCM, response time, temperature variation, and thermal degradation are the criteria for evaluating both PCMs. It is found that a higher temperature is obtained for SS-PCM, compared to SLPCM, after the heating cycle. A better thermal response is achieved by the considered SS-PCM compared to the chosen SL-PCM. Finally, this study aids the selection of SS-PCM and SL-PCM for a particular thermal storage-based application.