We report a theoretical analysis on the stereo- and regiochemical behavior of the S- and N-based group 4 heterocenes-developed by Ewen, Elder, and Jones-in the polymerization of propene to isotactic polypropylene. Somewhat surprisingly, the polymers produced with the N-based heterocene were shown to present a sensibly smaller amount of regiomistakes and a higher amount of stereomistakes relative to the S-based heterocene. To clarify the different mechanistic behavior of these structurally rather similar catalysts, we compared their stereo-and regioselectivity, and we extended our modeling to the corresponding all-carbon bis(indenyl)-based catalyst. Our analysis supports most of the ideas proposed by Ewen, Elder, and Jones to rationalize the experimental behavior.