In general 300mm and 400mm f2.8s are for sports first, wildlife 2nd.
500, 600, 800 are for wildlife first, and sports/other applications 2nd.
I have a 400mm f2.8 IS (the first one) and a 300mm f4, plus both vIII extenders. This gets me a wide range of telephoto options, from 300mm to 1280 (equiv. on the 7D)
The 600mm f4 II looks astonishing, but waaaay past my budjet.