Electropneumatic actuators are more and more used in industrial applications. Due to nonlinear phenomena, for many applications, high accuracy can only be obtained when using nonlinear control laws. These strategies allow to get high performances but often need the knowledge of all state variables. Then, there is a real interest to design observers in order to estimate state from the measurements of only one of the chamber pressures in the actuator and carriage position. This paper focuses on a comparison between two nonlinear observers for a half-meter stroke electropneumatic unsymmetrical cylinder controlled by two three-way servodistributors: a high gain observer and a sliding mode one. Simulations are made with a fluid power systems dedicated software, AMESim.