The relative energies, binding energies, and adsorption energies of three CuSi4 and two CuSi6 clusters have been computed in the fixed-node diffusion Monte Carlo (FNDMC), CASSCF, and B3LYP DFT methods. These results are compared with the earlier Hartree-Fock (HF) and B3LYP DFT investigations of these systems by two of us [C. Xiao and F. Hagelberg, J. Mol. Struct.: THEOCHEM 529, 241 (2000)]. The very close energy level spacing of the isomers under consideration confirms the previous work of Xiao and Hagelberg. The FNDMC results show some qualitative discrepancies with B3LYP DFT, and HF findings. They also confirm the appropriateness of the B3LYP DFT method for the prediction of the most stable CuSi4 isomer, while the CASSCF method compares more favorably with FNDMC for adsorption energies than B3LYP DFT.