The comparative study of the DC parameters and RF power stability of nitride-based conventional HFETs and Metal-Oxide-Semiconductor HFETs (MOSHFETs) is presented. The average lifetime under DC stress is estimated to be as high as 2.5 years at room temperature. Under the large-signal RF stress, the gate leakage current of conventional HFETs increases significantly with time showing faster degradation as compared to DC stressing. MOSHFETs demonstrate superior RF performance stability, which perfectly correlates with the DC stability data. It is shown that in conventional HFETs, the combination of the self-heating and positive dynamic gate bias leads to the defect accumulation in the AlGaN barrier under the gate. In MOSHFETs, the absence of the gate leakage is a key to stable RF performance.