Future lunar reactor power systems are anticipated to solve both near- and far-term human exploration mission needs. Early systems will likely be in the 10–50 kWe class with follow-on systems that could be 100 kWe or greater. There is similar demand for reactor power systems on Mars, and benefits can be realized by using common technologies and design strategies across the two mission applications. Several recent projects have helped to mature the design concepts and requisite technologies. During the NASA Constellation Program, following in the footsteps of Project Prometheus, a NASA-DOE team developed a low-cost, low-risk 40 kWe Fission Surface Power concept that was corroborated by extensive component and system testing. A follow-on effort to investigate kilowatt-class fission power systems, later named Kilopower, was triggered by interest to identify alternatives to radioisotope power systems for planetary science missions. The Kilopower project culminated in a 1 kWe-class reactor prototype test at the Nevada National Security Site in 2018 and designs for space reactor power systems up to 10 kWe. The design options for future reactor flight systems are abundant, and the decision process for selecting the right technologies is fraught with complications. To guide the process, an analysis is presented that compares surface reactor power system options for power levels from 25 to 200 kWe. The study explores both low- and high-temperature reactors, three different power conversion technologies, and two different shielding approaches.