In this paper two recent analyses are reported which demonstrate advantages of a U233 fueled thermionic fuel element (TFE) compared to 93 w/o U235, and that application (mission) has broad latitude in how space power reactor systems could or should be optimized. A reference thermionic reactor system was selected to provide the basis for the fuel comparisons. Both oxide and metal fuel forms were compared. Of special interest was to estimate the efficiencies of the four fuel forms to produce electrical power. A figure of merit (FOM) was defined which is directly proportional to the electrical power produced per unit uranium mass. In a TFE the average electrical power produced is proportional to the emitter surface area (Esa), hence the ratio Esa/Mu was selected as the FOM.Results indicate that the choice of fuel type and form leads to wide variations in critical and system masses, FOM values, and system total power. While not fully comprehensive, the results suggest that U233 oxide offers very significant advantages over U235 oxide in fuel mass ( 52% less), FOM ( 58% higher), system mass (16% lower), but produces less electrical power in the reference system. The advantage is due to superior nuclear properties of U233.The data are presented so as to highlight the parameters which the user/designer may select to optimize and the favorable consequences of treating the lattice spacing as a parameter in the U233 oxide system.