The highly respected German website Slashcam just published some initial thoughts about the R5 and R6 which I think are worth sharing here. What follows are a few excerpts, imperfectly translated via Google. :))
[…] At first glance, the EOS R6 looks a bit slimmer in addition to the 8K RAW specifications of its big sister. In practice, however, the lower number of sensors may even prove to be advantageous for the dynamics. The EOS R6 therefore has a sensor size of 6.6 µm, while the EOS R5 has to accommodate 8192 horizontal pixels in the same sensor width, which results in a sensor size of 4.4 µm. The area of an R5 sensor can therefore only cover half of the photons of an R6 sensor, which corresponds to around an aperture stop. The R6 should therefore clip about an aperture stop later than the R5, as long as Canon has not installed other tricks like DualGain (aka DGO on the C300 MkIII) here. However, we would rather doubt the latter.
[…] Canon itself also says that the sensor of the EOS R6 is very similar to that of the DSLR professional EOS-1D X Mark 3. And that bodes well not only for the dynamics. A 5.5K sensor also offers a fairly pragmatic resolution for 4K downscaling, which has already proven almost flawless up to 60p in the 1D X Mark III. We still lack the last confirmation, but the 4K modes above 30p in the R5 are likely to drop somewhat in terms of quality, since here it is no longer the case that each individual sensel is downsampled, but methods such as binning will be used. With the R6, on the other hand, it is to be expected that the 4K quality up to 60p will be consistently sampled down from 5K.
[…] The difference between 12 bit RAW and good 10 bit 4: 2: 2 is also not earth-shattering in our eyes and 8K does not necessarily appeal to every filmmaker. We are therefore not sure whether the R6 is not even the bigger hit among filmmakers, which now has to be topped by the competition. We already suspected this in our test conclusion on the EOS 1D X Mark III: “With the EOS-1D X Mark III, Canon shows the competition (and above all, Panasonics S1H) that it is not only theoretically able to use a camera with a modern design The image quality of 5.5K RAW with up to 60fps is just as flawless as the 4K HEVC scaled down in the camera in 10 bit 4: 2: 2 signal.”