Among various modeling methods for DC-DC converters introduced in the past two decades, the state-space averaging (SSA) and the circuit averaging (CA) are the most general and popular exhibiting high accuracy. However, their deduction approaches are not entirely equivalent since they incorporate different averaging processes, thus yielding different small signal transfer functions even under identical operating conditions. Some researches claimed that the improved SSA can obtain the highest accuracy among all the modeling methods, but this paper discovers and clearly verifies that this is not the case. In this paper, we first review, study these two modeling methods for various DC-DC converters operating in the discontinuous conduction mode (DCM). We also streamline the general model deriving processes for DC-DC converters, test and compare the accuracy of these two methods under various conditions. Finally, we provide a selection strategy for a high accuracy modeling method for different DC-DC converters operating in DCM and verified by simulations, revealed necessary and beneficial for designing a more accurate DCM closed-loop controller for DC-DC converters, thus achieving better stability and transient response.