Cubic A2B2O7 oxides with A standing for a rare-earth element and B for a transition or main block metal have been systematically studied for their frequently exotic crystallographic and electronic properties. The majority of A2B2O7 compounds crystallize in a cubic pyrochlore or defect-fluorite structure. In the proposed experiment we focus on Er2B2O7 oxides with B = Zr and Ir, adopting defect-fluorite and pyrochlore structures, respectively. Despite structural differences, both the zirconate and iridate show surprisingly similar spin-glass-like magnetic transitions at low temperature. We aim to use muons to confirm and probe the spin-glass ground-state of these materials to help clarify how different frustrating processes (structural disorder in the fluorite structure vs. geometric frustration in the pyrochlore) can lead to similar magnetic properties.