This paper presents an analytic approach to obtain the optimum source and load impedance values for different criteria of maximum power gain, linearity and efficiency. A comparison of two identically rated commercially available 25W RF power transistors in silicon LDMOS and GaN-on-Si HEMT reveals that GaN HEMTs are relatively immune to optimization criteria popularly used by RF amplifier designers, viz. maximum linearity, power gain or power added efficiency, in comparison to silicon LDMOS. This phenomenon is attributed to the higher values of feedback capacitance and output conductance of the GaN HEMTs compared to silicon LDMOS. At a lower frequency of 1.5 GHz, LDMOS devices show higher small-signal gain than its GaN-on-Si counterparts, likely due to smaller parasitic in their device design and fabrication process.