I’ve tested and compared these two sensors, and the image quality of the R6 and R8 should be very similar, with the extra 4MP of the R8 being the main talking point (20.1MP vs 24.2MP).
The R6 features in-body 5-axis stabilisation, with a maximum compensation of 8 stops. The R8 doesn’t have a stabilisation mechanism on the sensor, so you’ll have to rely on lenses with IS.
The R8 may sit a category below the R6, but it has the latest version of Canon's AF software and algorithm. Expect better performance in low light especially.
The R6 has a max. shutter speed of 1/8000s. The R8 is slower with the mechanical shutter (1/4,000s) but faster with the electronic shutter (1/16,000s).
The R8 receives most of the latest features Canon has added to other cameras in recent years such as breathing compensation, Dual Pixel RAW, Focus Stacking and more.
The R6 is larger and heavier, with a taller grip and more advanced weather sealing. The R8 has good ergonomics and a decent grip, but lacks a few features professional photographers may seek.
The R8 viewfinder is not up to the standard of the R6 viewfinder: it is smaller, has less resolution and less magnification, but reaches the same frame rate of 120Hz.
The R6 has two SD card slots (UHS-II) with the dedicated compartment found near the grip. The R8 has one slot (SD UHS-II), and the card goes into the battery compartment.