The thermodynamic performances of the two different integrated solar combined cycle (ISCC) systems are compared in this paper. Different from the previous comparison researches of ISCC systems based on different solar energy collecting technologies, the goal of this paper is to compare the integration characteristics of two different configurations of integrating concentrated solar energy into a gas turbine combined cycle (GTCC) system based on the same solar collector system. For the first kind of integrated solar gas-steam combined cycle system (ISCC1), the solar energy is introduced to the topping cycle of the gas-steam combined cycle system, while for the second kind of integrated solar gas-steam combined cycle system (ISCC2), the solar energy is introduced to the bottoming cycle of the GTCC system. The detailed system models are developed and their thermal performances are compared under different conditions. For ISCC1, the solar-to-electricity efficiency is higher than that of ISCC2 at the design condition when both the direct normal irradiation and ambient temperature are high due to more efficient energy conversion to electricity. However, the ISCC2 offers the advantages of higher solar-to-electricity efficiency and more solar power output when both the direct normal irradiation and ambient temperature are low. Two ISCC systems are good for energy saving, the ISCC1 consumes 4.412 × 108 kg of fuel a year, which is 2.803 × 106 kg less than that of ISCC2, and the ISCC1 has an annual solar-to-electricity efficiency of 23.93%, 0.88% higher than that of ISCC2. Detailed daily and monthly simulation results show that two systems have advantages of saving energy, and the simulations results show the obvious effects of different solar energy integration modes on the overall IGCC system performance. The achievements of this paper can offer valuable references for the design and operation optimization of ISCC system.