The magnetic viscosity coefficient in Y x Sm 1 - x Co 3 Cu 2 alloys, with x=0, 0.25, 0.5, 0.75, and 1, was determined from measurements of the magnetization reversal relaxation close to the coercive field using a vibrating sample magnetometer and from the coercive field relaxation by the sweep-rate method using a pulse-field magnetometer, respectively. The coercive field and the magnetic viscosity coefficient, as determined from both methods, decrease with increasing yttrium concentration. However, the magnetic viscosity coefficient obtained from magnetization relaxation is always smaller (about six times) than that obtained by means of pulsed field measurements. These results are explained on the basis of different time windows of the measurements, proving different flat distributions of energy barriers due to a complex distribution of activation energies.