We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO₃ (LMO) and La_0.7Ca_0.3MnO₃ (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the subpicosecond dynamics of colossal magnetoresistance manganites in terms of a phenomenological three-temperature model describing the energy transfer between the electron, lattice, and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long-term relaxation due to film-to-substrate heat diffusion and then develop a modified three-temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the band gap in LMO. In addition, we also model the nonthermal subpicosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.