A mobile manipulator, composed of a manipulator mounted on a vehicle, is a very useful system to achieve tasks in dangerous environments. Unfortunately it is a very complicated system to model and control. One solution is to control the manipulator and the vehicle independently with advanced control schemes. In particular motions of the vehicle on uneven terrains induce disturbances on the arm behaviour that must be either taken into account or rejected by the arm control scheme. The aim of this paper is to compare the robustness of hybrid position/force and external force control structures with respect to external disturbances, for a manipulator mounted on a vehicle. The principles of both control structures are recalled. Some theoretical analysis tends to prove the superiority of the external control structure. This is confirmed by experiments with our real testbed. Results are presented in the form of graphs.