In the literature, two different frameworks exist for describing the rheology of solid/liquid suspensions: (1) the “viscous” framework in terms of the relative suspension viscosity, η_r, as a function of the reduced solid volume fraction, f=f_m, with f_m the maximum flowable packing fraction, and (2) the “frictional” framework in terms of a macroscopic friction coefficient, μ, as a function of the viscous number, I_v, defined as the ratio of the viscous shear to the wall-normal particle stress. Our goal is to compare the two different frameworks, focusing on the effect of friction between particles. We have conducted a particle-resolved direct numerical simulation study of a dense non-Brownian suspension of neutrally buoyant spheres in slow plane Couette flow. We varied the bulk solid volume fraction from f_b ¼ 0:1 to 0.6 and considered three different Coulomb friction coefficients: μ_c ¼ 0, 0.2, and 0.39. We find that η_r scales well with f=f_m, with f_m obtained from fitting the Maron–Pierce correlation. We also find that μ scales well with I_v. Furthermore, we find a monotonic relation between f=f_m and I_v, which depends only weakly on μ_c. Since η_r ¼ μ=I_v, we thus find that the two frameworks are largely equivalent and that both account implicitly for Coulomb friction. However, we find that the normal particle stress differences, N₁ and N₂, when normalized with the total shear stress and plotted against either f=f_m or I_v, remain explicitly dependent on μ_c in a manner that is not yet fully understood.