A partial cation exchange has been used to synthesize Cu_2S-CdS and Ag_2S-CdS nanocrystal heterostructures, with two very different morphologies. Cu^+ cation exchange takes place preferentially at the ends of CdS nanorods, Cu_2S segments grow into the nanorod from both ends. Ag^+ exchange is non-selective, Ag_2S islands nucleate and grow over the entire surface of the nanorod. This leads to very different patterns, striped Ag_2S-CdS superlattice with several equidistant Ag_2S segments in a CdS nanorod, and an asymmetric Cu_2S-CdS heterostructure with Cu_2S segments at the ends of the CdS nanorod. We use first-principles calculations to explain the formation patterns in the two nanostructures, and explore their elastic and electronic properties. Comparison of chemical and elastic contributions to the interface formation energy for the Cu(Ag)_2S-CdS shows that the relative stability of the interfaces determines the nucleation of Cu(Ag)_2S and the resulting morphology. Furthermore, since two end facets of CdS nanorod are not crystallographically equivalent a controlled asymmetric nucleation of Cu_2S can occur.