This work aims to compare beta-type Stirling engine performance (GPU-3 [ground power unit]) driven by rhombic and crank mechanisms. A modified non-ideal adiabatic model accounting for different frictional and thermal losses was adopted in this study. After validating the current model with engine experimental data, different scenarios of operating conditions including heater temperature, cooler temperature, charge pressure and engine speed were investigated. The results revealed that rhombic drive mechanism generates 32% more power and provides 20% more efficiency than crank mechanism at normal operating conditions. However, at low hot end temperature (300 degrees C) and high charge pressure (50 bar) crank drive mechanism tends to slightly generate power more than rhombic drive mechanism at lower engine speeds. At low hot end temperature (300 degrees C) and charge pressure (10 bar) both mechanisms cannot deliver any positive power. Higher power loss is recognized in crank drive mechanism at higher speeds due to increased pumping and gas spring hysteresis losses. This study highlights a wide analysis opportunity for designers and researchers of GPU-3 Stirling engine for further optimization.