This paper deals with a fractional system composed of a storage I-element and a fractance. The fractance is approximate by a network of 4 identical RC cells arranged in gamma and a purely capacitive cell, thus defining a rational system. When compared to each other, the dynamic behaviors of the fractional and the rational system show an excellent superposition of frequency and time-domain responses. Moreover, the robustness of stability margins obtained with both systems is illustrated versus variations of the I-element.