Two low-energy minima of (H2O)(21) with very different H-bonding arrangements have been investigated with the B3LYP density functional and RIMP2 methods, as well as with the TIP4P, Dang-Chang, AMOEBA, and TTM2-F force fields. The AMOEBA and TTM2-F model potentials give an energy ordering that agrees with the results of the electronic structure calculations, while the TIP4P and Dang-Chang models give the opposite ordering. Insight into the role of many-body polarization for establishing the relative stability of the two isomers is provided by an n-body decomposition of the energies calculated using the various theoretical methods.