Semi-active magnetorheological (SA MR) dampers seem to represent the easiest way to materialize the concept of smart devices for semi-active structural control. SA MR dampers can be utilized as reactive force generators, when the control algorithms adopted to drive the devices are derived in the framework of control theory, or as smart dampers, when the real-time change of their mechanical properties is aimed at providing, at any time, the optimal amount of damping in a structure. The two approaches have different requirements in modelling the SA MR devices. Based on an experimental campaign on two prototype devices manufactured in Europe, the present paper compares the effectiveness of numerical models presented in the literature and analyses the response time and the dissipative capabilities of such devices.