1. Shoot some side-by-side, low light, indoor portrait (or some complex subject). For both cameras, use the same exposure time (1/200...1/50), F-number (F2...F4) and amount of light. Only vary the ISO if the sensitivity is different, in order to get photos of the same relative brightness on the display.
DO NOT vary the amount of light that reaches the sensor, or the comparison is useless! The ISO doesn't vary the amount of light that reaches the sensor.
By low light I mean that in the context of these settings, the ISO should be 6400...25600 (okay, 3200 is acceptable at around 1/50 and F2...F2.8 ). If possible, use RAWs, do not use noise reduction of any kind and convert to JPEG without post-processing.
2. Check the ability of the cameras to (consistently and *properly*) expose back-lit (preferably indoors), half-body portraits, where you use the center AF point, focus on the face and then recompose (to check that it does exposure lock; it should, but...). You might also want to test with the auto-lighting-optimizer activated (for Canon).
3. Some low light shot(s) and then apply a bunch of fill light (/ shadows) to see what dark details can be recovered.
Hey, you've asked, so... thank you anyway