His sig says he already has a 5d3, so why get a 6d?
MStevens, why are you using a 40d when you have a 5d3? The 5D3 is better in all aspects. If it is a reach issue, you can also toss in a 1.4x iii on your 70-200. Even without a TC, the 5d3 already has similar reach as the 40d (1.6x crop vs 2.2x more pixels). If its is really framerate that you miss, I concur with Neuro that the 7D (more fps, larger buffer and CF) is the better choice.
Missed that. I'd pick a 5DIII over the 7D or 70D for sports. The AF is better, enough better that the keeper rate is higher than the 7D despite the slower frame rate. The IQ is equivalent at low ISO if you crop to the APS-C FoV, better if you don't need to crop and cropped at high ISO.
Unless you're looking for a second body to use along side the 5DIII, just use the 5DIII.