The a7siii shaping out to be a very compelling video camera option.
Videography wins for R5:
- Detail in 4kHQ due to oversampling from 8k
- Internal raw
- stabilization (I think - due to larger mount opening)
- Autofocus (I'm guessing, we'll have to wait to see for sure)
- Ergonomics (subjective)
Videography wins for a7siii:
- Dynamic Range
- Raw footage at a more usable resolution
- Higher framerate HD (oversampled 240 FPS 1080 will probably look awesome)
- raw video bitrate (16 bit!?!)
- no record limit (why you do this to us Canon?!?)
- no overheating (we'll see on this, but given you're dealing with 1/4 the pixels, I'd think it'd have a much easier time and this is going to be a dealbraker for the Canon for a lot of people doing interviews/docs/other longform stuff)
- less rolling shutter (probably, we'll see)
Overall, if you're primarily a video shooter, I'd think that the a7siii would be the camera to get. It ticks many of the boxes that drove me to pick up a used C300 that the R5 doesn't address (Canon, will you ever release a <$10k camera that can shoot a whole interview without hitting an artificial record limit in modes where it doesn't overheat). Pair this thing with an external recorder that has XLR inputs and you're looking at a package that offers a lot of what you get with a C500ii for 1/3rd the price. Of course, having to use an external recorder for a lot of the nicer modes is going to be a major drawback for certain genres of videographer. And I don't see it being a particularly compelling option for stills photographers, although I'm sure it would make very nice 12MP images (and at a rate of 120 frames per second).
***An additional thought***
Just occurred to me that if you were to use an external recorder with the R5/R6, you would bypass the record limit. So that somewhat diminishes that advantage on the part of the Sony (although to be fair, any video mode the R5/R6 is able to record for longer than 30 minutes without encountering thermal issues can be recorded internally by the a7siii, so the a7siii still holds a considerable advantage in this regard).