The main objective of this study is to compare and optimize two power-to-gas energy storage systems from a thermo-economic perspective. The first system is based on a solid oxide electrolyzer cell (SOEC) combined with a methanation reactor, and the second is based on a polymer electrolyte membrane electrolyzer cell (PEMEC) integrated into a Sabatier reactor. The first system relies on the co-electrolysis of steam and carbon dioxide followed by methanation, whereas the basis of the second system is hydrogen production and conversion into methane via a Sabatier reaction. The systems are also analyzed for being applied in different countries and being fed by different renewable and non- renewable power sources. Simulation results of both systems were compared with similar studies from the literature; the errors were negligible, acknowledging the reliability and accuracy of the simulations. The results reveal that for the same carbon dioxide availability (i.e., flow rate), the SOEC-based system has higher exergy and power-to-gas efficiencies, and lower electricity consumption compared to the PEMEC-based system. However, the PEMEC-based system produces 1.2 % more methane, also with a lower heating value (LHV) of the generated gas mixture that is 7.6 % higher than that of the SOEC-based system. Additionally, the levelized cost of energy (based on the LHV) of the SOEC-based system is found to be 11 % lower. A lifecycle analysis indicates that the lowest lifecycle cost is attained when solar PV systems are employed as the electricity supply option. Eventually, the SOEC-based system is found to be more attractive for power-to-gas purposes from a thermo-economic standpoint.