The subject of this paper is the mutual comparison of switching energy losses in cascode gallium nitride HEMT and silicon "superjunction" MOSFET transistor, both designed for a maximum operating voltage of 650 V. For the purpose of analysis the transistor switching characteristics, the double pulse test method was implemented. Detailed computer simulation models developed in programs of the SPICE family were used. Data on transient turn -on and turn-off processes were generated by LTspice simulation tool, in a wide range of drain currents, using two different gate resistance values for driving the transistors under test. The obtained results indicate superior switching characteristics of gallium nitride devices in comparison to silicon components, especially during the high drain current transistor operation. During the one transistor switching cycle, the total energy losses in the GaN HEMT were simulated, for a drain current of 30 A, and found to be five to eight times lower when compared to tested Si MOSFET transistor.