Various structural possibilities for the Si2C4 and Si4C2 clusters are investigated by employing a basis set of triple-zeta plus polarization quality; electron correlation is generally accounted for by second-order Moller-Plesset and, in certain instances, by higher-order perturbation (CASPT2) approaches. The building-up principle recently suggested from an analysis of Si3C3 clusters is found to be fully operative for Si2C4 and Si4C2 clusters. A comparison of the structure and stability of various geometrical arrangements in the series C-6, Si2C4, Si3C3, Si4C2, and Si-6 shows that linear and planar structures become rapidly less stable if carbons are replaced by silicons and that the three-dimensional bipyramidal forms become less favorable as soon as silicons are exchanged by carbons in the parent Si-6 structure. The effects can be rationalized in qualitative terms based on differences in silicon and carbon bonding.