Transport phenomena in a proton exchange membrane fuel cell (PEMFC) are highly coupled and complicated by multi-phase flow in the several porous components, electrochemical reaction and the three-dimensional geometry. Naturally, therefore, theoretical work is largely manifested in computational fluid dynamics (CFD). However, the comprehensive three-dimensional (3D) CFD method is often prohibitively time-consuming and, consequently, is not yet a suitable basis for a screening tool that operates under a wide range of design options and operating conditions. Mathematical models and solution procedures using simplified models with reduced dimensions have been proposed to address the issue of CFD time expense. Such approaches are computationally efficient, but no systematic study has been conducted to quantitatively assess the effects of the neglected dimensionality. In this paper we compare 3D CFD models, for a straight unit cell, with a hierarchy of reduced-dimensional models. We demonstrate that the 2+1D approach is currently the optimal choice as the basis for a tool to assist MEA and unit-cell design in the early stages of a design cycle.