A number of recent studies have explored the energetics of intermediate;sized clusters, such as small fullerenes. For example, the isomer energies of C-20 have proven difficult to ascertain as different theoretical methods give widely varying results. For cluster sizes around 20-30 atoms, it is not immediately clear whether a monocyclic, graphitic, or fullerene structure is energetically preferred. In the present work, the isomers of B12N12, a ring, a "graphitic" sheet, and a fullerene-like cage of four- and six-membered rings, are examined by theoretical means. The energies of the three isomers are calculated using Hartree-Fock theory and density functional theory (DFT) in local and gradient-corrected forms. The Dunning correlation-consistent basis sets are used, with geometries optimized up to the triple-zeta level and energies calculated at the quadruple-zeta; level. The effects of the basis set, as well as the effects of local versus nonlocal DFT, are discussed. The results are compared to those of similar studies of C-24, which is isoelectronic with B12N12.