Ionic species with a constant charge-to-size ratio (i.e. electrophoretic mobility) cannot be separated in electroosmotic or pressure-driven flow along microscale channels. In nanoscale channels, however, the enormous electric fields inside electrical double layers cause transverse ion distributions yielding charge-dependent mean ion speeds in the flow. Those ions with a constant charge-to-size ratio can thus be separated solely by charge (or equivalently, size) in nanofluidics. Here we develop an analytical model to optimize and compare the separation of such ions in nanochannel chromatography and nanochannel electrophoresis in terms of selectivity, plate height and resolution. Both planar and cylindrical geometries are considered. It is found that in nanoscale channels chromatography yields a larger selectivity and a larger minimum reduced plate height than electrophoresis does. The maximum resolution is, however, comparable between these two nanofluidic approaches, where the optimal channel half-height or tube radius is within the range of 1-10 times the Debye length. Our results also suggest that cations can be better separated in nanofluidics than can anions.