Collision resolution is one of the key elements in a discrete element method algorithm for modeling granular flows. Several collision models have been proposed for this process. The hard-particle collision approach is typically used for dilute systems, or for those in which the assumption of binary and instantaneous particle-particle contact remains valid. As the solids fraction increases, however, multiple, enduring collisions can occur and a soft-particle approach is more appropriate for resolving the collision dynamics. In this work, the delineation between dilute and dense systems and the suitability of contact models are explored for a range of solid fractions. Stress results for two-dimensional shear flow simulations are compared using several collision models including an event-driven hard-particle model, a hysteretic spring soft-particle collision model following Walton and Braun [J. Rheol. 30, 949 (1986)], and a hybrid hard-particle-with-overlap model following Hopkins and Louge [Phys. Fluids A 3, 47 (1991)]. Results show that stresses are accurately predicted for a range of solids fractions, coefficients of restitution, and friction coefficients by both the hard-particle-with-overlap and soft-particle models so long as a sufficiently large loading stiffness is used for the soft-particle model. Additional results investigating the accuracy of the collision models and the amount of collisional overlap are presented as functions of the simulation time step and model parameters.