Using MC simulations based on various models composed of spherical scatterers, cylindrical scatterers, and birefringent interstitial medium, we compare the polarization parameters extracted, respectively, with the $4\xd74$ and $3\xd73$ Mueller matrix decompositions in the forward scattering direction. The simulations are backed up by experiments when suitable samples are available. The simulated and the experimental results show that the depolarization and linear retardance obtained from the $3\xd73$ and $4\xd74$ Mueller matrix decompositions are usually not the same quantitatively, but display similar qualitative relations to changes in the microstructure of the sample, such as the density, size, and orientation distributions of the scatterers and birefringence of the interstitial medium. For depolarization, the $3\xd73$ Mueller matrix decomposition results in bigger contributions from the spheres and a smaller contribution from the cylinders. For retardance, both decomposition methods can present the phase retardance, respectively, due to two types of anisotropy sources in the tissue models: birefringence in the ambient medium and the cylindrical scatterers. Compared with birefringence, the retardance deviation due to cylinders using the $3\xd73$ Mueller matrix is more apparent. Additionally, it should be noted that the decomposition of the $3\xd73$ Mueller matrix for spheres can generate a singularly higher linear retardance, which can be confused with the real anisotropic microstructure or optical birefringence. We also carried on the above comparison of these two decomposition methods in the backward scattering detection and obtained similar conclusions. Therefore, the parameters for the $3\xd73$ Mueller matrix decomposition method follow the same qualitative trends as the corresponding results for the $4\xd74$ Mueller matrix decomposition method, although quantitatively, the two sets of parameters could be significantly different. Images of the corresponding $3\xd73$ and $4\xd74$ decomposition parameters should display similar patterns but different contrasts.