Solar cells based on metal halide perovskites often show excellent efficiency but poor stability. This degradation of perovskite devices has been associated with the migration of mobile ions. MAPbBr₃ perovskite materials are significantly more stable under ambient conditions than MAPbI₃ perovskite materials. In this work, we use transient ion drift to quantify the key characteristics of ion migration in MAPbBr₃ perovskite solar cells. We then proceed to compare them with those of MAPbI₃ perovskite solar cells. We find that in MAPbBr₃, bromide migration is the main process at play and that contrary to the case of MAPbI₃, there is no evidence for methylammonium migration. Quantitatively, we find a reduced activation energy, a reduced diffusion coefficient, and a reduced concentration for halide ions in MAPbBr₃ compared to MAPbI₃. Understanding this difference in mobile ion migration is a crucial step in understanding the enhanced stability of MAPbBr₃ versus MAPbI₃.