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Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic Cr7 Ni and Cr7 Ni-Cu-Cr7 Ni molecular clusters. In Cr7 Ni the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant J, while Cr7 Ni-Cu-Cr7 Ni is composed of two Cr7 Ni 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 μ0 H < 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 Cr7 Ni and T ~ 8 K for Cr7 Ni-Cu-Cr7 Ni, the two compounds have distinct additional features: Cr7 Ni shows a shoulder in λ(T) for T < 8 K, while Cr7 Ni-Cu-Cr7 Ni 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 Cr7 Ni 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 Cr7 Ni-Cu-Cr7 Ni 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. [ABSTRACT FROM AUTHOR]