Due to the potential need for ultra-wide input voltage range power factor correction (PFC) circuit, one may consider Buck-boost converter as a PFC circuit (PFC Buck-boost) instead of Boost converter (PFC Boost). The voltage step-up/down freedom of Buck-boost converter allows a lower output voltage and smaller duty cycle compared to Boost converter. In addition, indirect input/output path limits the startup inrush current of PFC Buck-boost. Unlike PFC Boost, PFC Buck-boost doesn't need an inrush current limiting circuit, which allows a lower cost PFC circuit by applying Buck-boost converter. This paper conducts a comparison study of PFC Boost and PFC Buck-boost for a two stage system (PFC stage in cascade with an isolated DC-DC converter). Transient mode PFC is selected in this study for low power applications. Two 100W power supply prototypes are made for this comparison. The results show that PFC Buck-boost is able to provide a higher efficiency than PFC Boost under low input voltage with fewer power stage circuit components.