A direct solution to Global Warming would be to reflect a part of sunlight back into Space. This paper is a system tradeoff study of two concepts being evaluated as long-endurance high-altitude reflectors. The first is a centrifugally stretched high altitude solar reflector (CSHASR). This avoids the drag penalty of continuous translational flight compared to fixed-wing towed ‘Flying Carpet’ concepts. It consists of a large sheet or several segments thereof, assuming the shape of a Frisbee (convex upper surface with Coanda effect over the lip, concave lower surface terminating in vertical lip). This offers a way to stretch a thin sheet without the expected flag instabilities of translational flight. The CSHASR rotates as one of 4 such ‘frisbees’ held together by an ultralight quad structure. Each spins in reaction to a solar-powered rotor that provides lift augmentation and thrust directional control. A second concept is an Aerostatically Balanced Reflector (ABR) sheet, held up by hydrogen balloons. A set of co-axial counter-rotating rotors provides lift augmentation, trim, directional control and the ability to conduct an annual migration as the summer shifts from the northern to southern hemisphere and back. This concept also offers the ability to hold up the reflector at arbitrary orientations to achieve maximum reflection, normal to the slanted rays of the polar summer sun. The paper goes into the detailed aerodynamics and material/structural concept calculations in order to perform a tradeoff study and reports the results. At present the ABR appears to be superior in most respects to the CSHASR, but this result is being refined. For the full paper we expect to present initial experimental results from a ground wind facility.