Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic Cr₇Ni and Cr₇Ni-Cu-Cr₇Ni molecular clusters. In Cr₇Ni the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant J, while Cr₇Ni-Cu-Cr₇Ni is composed of two Cr₇Ni linked by a bridging moiety containing one Cu ion, that induces an inter-ring ferromagnetic interaction J' ≪ J. The longitudinal muon relaxation rate λ collected at low magnetic fields μ₀H < 0.15 Tesla, shows that the two systems present differences in spin dynamics vs temperature. While both samples exhibit a main peak in the muon relaxation rate vs temperature, at T ∼ 10 K for Cr₇Ni and T ∼ 8 K for Cr₇Ni-Cu-Cr₇Ni, the two compounds have distinct additional features: Cr₇Ni shows a shoulder in λ(T) for T < 8 K, while Cr₇Ni-Cu-Cr₇Ni shows a flattening of λ(T) for T < 2 K down to temperatures as low as T = 20 mK. The main peak of both systems is explained by a Bloembergen-Purcell-Pound (BPP)-like heuristic fitting model that takes into account of a distribution of electronic spin characteristic times for T > 5 K, while the shoulder presented by Cr₇Ni can be reproduced by a BPP function that incorporates a single electronic characteristic time theoretically predicted to dominate for T < 5 K. The flattening of λ(T) in Cr₇Ni-Cu-Cr₇Ni occurring at very low temperature can be tentatively attributed to field-dependent quantum effects and/or to an inelastic term in the spectral density of the electronic spin fluctuations.