This paper compares the performance of SiGe (Si0.8Ge0.2) and skutterudite segmented thermoelectric unicouples (STUs) at a hot side temperature of 973 K and cold side temperatures of 300, 573 and 673 K and for the same total length and cross sectional dimensions of the p-leg. The area of the n-leg and lengths of and the interfacial temperatures between segments of various materials in the STU legs are determined using a global optimization methodology. Results indicate that the STUs could potentially achieve peak efficiencies of 7.8% and 14.7% when operated at a cold side temperature of 573 K, typical of that in current radioisotope thermoelectric generators (RTGs), and 300 K, respectively. These efficiencies are 55% and 99% higher, respectively, than for SiGe at the same temperatures. However, due to the higher density of skutterudite, the electrical power densities corresponding to the peak. efficiencies of the STUs are 39 and 109 W-e/kg versus 92 and 232 W-e/kg for SiGe at cold side temperatures of 573 K and 300 K, respectively. On the other hand, the heat inputs and heat rejection powers for the STUs are 42-55% and 39-53%, respectively, of those for SiGe at the peak efficiency and 70-75% and 67-77% of those of SiGe, respectively, at the peak electric power density. Therefore, when used in RTGs, STUs could halve the (PuO2)-Pu-238 fuel mass and the radiator area, while operating at >45% higher electrical power density (>7 W-e/kg) than SiGe in current RTGs (similar to5.5 W-e/kg).