With the large-scale use of intermittent renewable energy worldwide, such as wind energy and solar energy, energy storage systems are urgently needed and have been rapidly developed. Technologies of compressed gas energy storage (CGES) and liquefied gas energy storage (LGES) are playing an important role, and air has been commonly used as working fluid. CO2 is another potential working fluid and attracting more and more attention due to the rise of CO2 capture and utilization. However, it is still unclear which is the better working fluid. This paper comparatively analyzed the performance of CGES and LGES systems using air and CO2 as working fluids. Both diabatic and adiabatic CGES are considered. Simulation results show that except diabatic CGES systems, using CO2 could achieve a similar or even higher round-trip efficiency than using air. In addition, the use of CO2 instead of air as a working fluid has additional advantages, such as a lower storage temperature can be achieved at the same storage pressure for the adiabatic CGES system; and a higher condensing temperature can be achieved at the same condensing pressure for the LGES system, which can benefit the system design and operation.