The 600 II is sharper than the 400 II + 1.4x, the 600 II + 1.4x is sharper that the 400 II + 2x, and the 600 II + 2x is 1200mm, still quite sharp, and will AF on the 5DIII after the firmware update due in April.
But...the 600mm is on the long side for sports. I shoot birds/wildlife a lot, so for me the 600 II was the best choice. If you shoot sports more than wildlife, the 400 may be better - it can be made longer with TCs. Do keep in mind there's an AF speed penalty with TCs - a 50% drop with a 1.4x and a 75% drop with a 2x. If you plan to mostly use an extender, get a longer lens (the only current exception is the 600 II, since that plus the 1.4x beats the 800/5.6, and the 600 II + 2x beats the 800 + 1.4x).
Once my kids are old enough for field sports (2-3 years, I guess), I can see adding a 300/2.8 II to my kit.