[Klaus]

I think the only aspect to keep in mind here is the simple fact that you can at least get decently fast 600mm zoom lenses for reasonable money.
If I had to choose between the Canon RF 800mm f/11 and the Sony 200-600mm f/5.6-6.3, I, for one, would have a clear preference.
Of course, that's irrelevant within the Canon scope, but then Canon is a choice as well.