A very nice problem to have I know, but I have an important sporting event to shoot in April and have decided to rent a EF 600mm f/4L IS II USM or EF 400mm f/2.8L IS II USM.
As it is a fast moving sport (think soccer/football/rugby) in an open air stadium, I am very tempted to couple my 1D X with the 400 f/2.8L IS II USM especially as a Group A lens it makes the most of the 1D X's AF system, and will perform better in low light (thought the entire pitch will be very well flood lit. But (always a but isn't there!), the extra reach of the EF 600mm f/4L IS II USM could be great to have. Both are version II.
I am not new to sports, but have never owned or used one of the very big whites so I do appreciate any input from those that do/have.