People often say (and they are very much right) that it is more important to select your lenses (or lens system) than the camera. You will end up changing bodies several times, but you will keep your investment in lenses long term. When I started, this seemed absurd, but now I can say it is absolutely true.
My investment in APS-C format Sony lenses is not extensive and has no relevance whatsoever to my possible choice of a full frame Sony body. My involvement with Sony mirrorless just gives some emotional momentum towards considering a full frame Sony mirrorless body. I have friends with full frame Sony mirrorless, and understanding the Sony lineup somewhat, helps me to put Canon cameras in perspective.
This camera is $2800 with 42 megapixels. 399 focus points In body image stabilization. 3.69m pixel EVF. 3 inch 2.4m pixel rear screen No flash
Despite the same pixel count, the III has an updated and better sensor, what they call a "back illuminated BSI" sensor. It looks like neither of these have in camera flash, ticking that all important box. So for the extra money, the III gets you a better sensor and much better autofocus.
Canon was late to the mirrorless party, with Sony leading the charge, but it is clear now that Canon is jumping on the mirrorless train with wild abandon.
I have a strong bias towards a Canon body by virtue of owning several high quality EF mount lenses. Canon offers a relatively inexpensive RP to EF adapter ($99) and clearly expects people to be doing what I intend to do, preserving their investment in Canon lenses and sticking with the Canon system.
Just not having to purchase the Sony 24-70 f/2.8 GM lens is $2300 I don't have to spend if I continue to use my Canon 24-70 f/2.8 L lens. That is nothing to sneeze at and is just one of several lenses.
One thing I absolutely want from a Canon mirrorless body is image stabilization. None of my lenses have it and it would be a great addition. Also my current DSLR is the 5D mark II with 24 megapixels. It would be attractive to get a mirrorless body with a denser sensor.
I have no interest in video and am wary of these "vlogger" oriented cameras for what I like to do. Any video features I get are nice "bonuses" to know about and have in case my interests shift someday, but in no way drive my decision making.
The R5 uses the touch screen for menus and almost everything.
The Strong point of the Sony A7 cameras is their autofocus, and the ability of the cameras to shoot fast bursts and such. Like video, these are not things I care a lot about. Yet most sports photographers are still using Canon. There are differences in layout and ergonomics, which generally boil down to individual preference.
A complaint about the R5 is that the low light (high iso) noise performance is not as good as Canon claims it is. They hide it by doing noise reduction in the camera (doing what people think is a good job). But the claim is that this is "baked into" the RAW file.
Some people don't want more than 45 megapixels.
I find this comment interesting:
I am a former Canon user but switched when I got tired of waiting for Canon to make a meaningful update to the 5D2. I shoot a Sony a9 and a7RIV. In my opinion, the Canon R5 is clearly a better all around camera than the a7RIV whereas the a7RIV is a slightly to moderately better landscape camera than the R5, and the a9 is a moderately better sports camera than the R5. Of course, the Sony A7SIII is a clearly better video camera than the Canon R5.And smart people point out it is all about lenses; bodies come and go.
A suggestion is that the R5 is like the 5D and the R6 is like the 6D, which is helpful if you know those DSLR cameras.
An interesting note I read that puts the "mere" 20 megapixel resolution of the R6 in perspective is that in 2002, Canon released the full frame 1Ds camera with 11.1 megapixels (and a price tag of $8000). This was considered to be the first time a digital camera had surpassed the resolving power of 35mm Kodachrome film.
Tom's Digital Photography Info / tom@mmto.org