Background: Quasifission and fusion-fission are primary mechanisms to prevent the production of superheavy elements. The recent experimental measurements reveal that the fusion-evaporation cross section in the 3n reaction channel of Ca-48 + (PU)-P-239 is 50 times lower than using Pu-244 as target nucleus [Utyonkov et al., Phys. Rev. C 92, 034609 (2015)]. However, the precise mechanisms of this remarkable isotopic dependence are not well understood. Purpose: To understand the experimental observation of the rapid decrease of stability of superheavy nuclei as the neutron number decreases, the theoretical studies of quasifission and fusion-fission in connection with experimental production for Z = 114 flerovium isotopes are required to investigate the possible differences in reaction mechanisms induced by these two targets. Methods: We propose an approach called TDHF+HIVAP to take into account both the evolution of dinuclear system and the deexcitation of compound nucleus, which combines the microscopic time-dependent Hartree-Fock (TDHF) method for the fusion and quasifission dynamics with the statistical evaporation model HIVAP for fusion-fission dynamics. Results: Fusion is observed for both reactions Ca-48 + Pu-239,Pu-244 with the side orientation of the deformed target nucleus, while quasifission dynamics is observed for the tip orientation. The nuclear contact times, masses, and charges as well as the kinetic energies of the fragments, and the mass-angle distribution strongly depend on the colliding energy, impact parameter, and deformation orientation. The quantum shell effect displays a crucial role in both the quasifission and the fusion-fission processes. The quasifission is considerably reduced and the survival probability is enhanced around one order of magnitude in the reaction using Pu-244 target as compared to the Pu-239 case. Conclusions: The studies by using TDHF+HIVAP method well account for the experimental observations and the present method clearly shows its applicability in the reaction mechanisms of quasifission and fusion-fission dynamics. The experimental and theoretical results encourage the use of neutron-rich targets for the production of new superheavy elements.