This paper provides a brief presentation and a useful comparison between two nonlinear observers Extended Kalman Filter (EKF) and sliding mode observer (SMO). Both can be used for moderate-accuracy attitude determination systems for Low Earth Orbit (LEO) Earth-pointing spacecraft (s/c), which is typically achieved using Gyroscopes, Earth, and Sun sensors for attitude sensing. The use of these observers provides a substitute for the yaw data in case of the s/c eclipse periods or limited field of views. The nonlinear observability for this system is analytically investigated via the calculation of Lie derivatives to check the possibility of the system states estimation. The performances of both observers are presented, the SMO stability is proved and the SMO enhanced estimates are shown by simulation.