Propylene polymerization using unsymmetrical, ansa-metallocene complexes Me2Y(Ind)CpMMe2 (Y = Si, C, M = Zr, Y = C, M = Hf) and the co-initiators methyl aluminoxane (PMAO), B(C6F5)(3), and [Ph3C][B(C6F5)41 was studied at a variety of propylene concentrations. Modeling of the polymer microstructure reveals that the catalysts derived from Me2Si(ind)CpZrMe2 and each of these co-initiators function under conditions where chain inversion is much faster than propagation (Curtin-Hammett conditions). Surprisingly, the microstructure of the PP formed was essentially unaffected by the nature of the counterion, suggesting similar values for the fundamental parameters inherent to two-state catalysts. The tacticity of PIP was sensitive to changes in [C3H6] in the case of catalysts derived from Me2C(ind)CpHfMe2 and PMAO, or [Ph3C][B(C6F5)(4)], but the average tacticity of the polymer produced at a given [C3H6] decreased in the order [Ph3C][B(C6F5)41 > PMAO. With B(C6F5)3, the polymer formed was more stereoregular, and its microstructure was invariant to changes in monomer concentration. The PIP pentad distributions in this case could be modeled by assuming that all three catalyst/cocatalyst combinations function with different values for the relative rates of insertion to inversion (A) but otherwise feature essentially invariant, intrinsic stereoselectivity for monomer insertion (alpha, beta), while the relative reactivity/stability (g/K) of the isomeric ion-pairs present seems to be only modestly affected, if at all. Similar conclusions can also be made about the published propylene polymerization behavior of the C-s-symmetric Me2C(Flu)CpZrMe2 complex with different counterions. For every counterion investigated, the principle difference appears to be the operating regime (A) rather than intrinsic differences in insertion stereoselectivity (a). Surprisingly, the ordering of the various counterions with respect to A does not agree with commonly accepted ideas about their coordinating ability. In particular, catalysts when activated with B(C6F5)3 appear to function at low values of Delta as compared to those featuring B(C6F5)(4) (less coordinating) and FAI[(o-C6F5)C6F4](3) (more coordinating) or PMAO (more coordinating) counterions where the ordering in Delta is MeB(C6F5)(3) < B(C6F5)(4) < FAI[(o-C6F5)C6F4](3) approximate to PMAO. Possible reasons for this behavior are discussed.