To compare stability in AC use of NbTi and Nb₃Sn superconductors, a critical quench margin of the superconductors has been defined. The comparison has been done based on the critical quench margin obtained by numerical calculations on conditions of AC operation in the AC field. The comparison has also been done on other conditions of DC operation in DC fields superposed by AC fields. Through these comparisons, the advantages of Nb₃Sn superconductors over NbTi ones have been found as follows: As for AC operation in AC field (commercial frequency, amplitude: 0.1-0.5T) in power apparatuses, such as fault current limiters and shunt reactors, the high current density type Nb₃Sn (J_c: around 4, 000A/mm² at 0.5T, total loss: around 20kJ/m³/cycle in a peak field of 0.5T, 50Hz) is more stable than NbTi. As for AC operation in an AC field (commercial frequency, amplitude: 0.5-1.5T) in power apparatuses, such as armature windings of superconducting generators and power transformers, the low AC loss type Nb₃Sn (J_c: around 2, 000A/mm² at 0.5T, total loss: around 2kJ/m³/cycle in a peak field of 0.5T, 50Hz) is more stable than NbTi. Furthermore, as for DC operation in a DC field (5T) superposed by AC field (amplitude: peak field of 0.1-1.5T, frequency: 2.5-350Hz), the high current density tpe Nb₃Sn is more stable than NbTi.