I shoot a lot of sports, and there's no substitute for the great full-frame optical viewfinder. But one of my gripes with the 1D X Mark III (and all predecessors) is the small AF area coverage for viewfinder shooting. So the subject has to enter a good ways into the frame before AF can latch on.
Switch to Live View shooting, and you get 100% coverage Vertical/90% Horizontal.
Viewfinder AF Area:
Live View AF Area:
For shots where you want the player's full height head to toe to fill the frame, their head (the desired AF point) can be outside of the Viewfinder AF area, so you get focus lock on the torso/shoulder instead.
Consequently, I occasionally switch to live view to get the player's heads entering the left or right edge of view; camera locks on.
But I really dislike looking at the rear LCD to cover action (my eyes can't focus that close anymore anyway
).
So from this perspective alone, I think an EOS-1D R, if it has a great
fast viewfinder, would be a good compromise as I would expect it to retain 90%+ AF Area coverage.