I'd like to say don't worry about the .6 loss (that was my initial fear) especially with a 400mm being 640mm on the 7D. You can crop to your hearts content and get that same exact photo as you would with the 7d only the image from the 5D MK3 won't break down. I should at some point demonstrate this as I may be one of the few with both cameras though not sure.
Hmm not my experience between the 5D2 (and 5D3 should be similar, other than maybe at the much higher ISOs) and 7D. The 7D definitely pulls in more reach (granted sometimes in action you have jussst a hint of tracking or motion blur etc and maybe you don't resolve to the 7D photosite, but whenever you do....).
That said, very tentatively since I haven't gotten a lot of proper opportunities with either, I think the 5D3 focuses better for soccer than the 7D. The 7D appears to act weirdly for soccer under certain lighting and heat wave conditions to the point that in one game late last summer I gave up and switched to my 5D2 for the rest of the game. OTOH my 7D focused really well for surfing at the Quiksilver Pro New York and one day there was horrible back lighting.
Maybe some of it is to do with the 7D seeming to have really large focus zone if you use a point with expansion and maybe it's too grabby for soccer and yet one point is not ideal either or, like many canon cams that came out for a few years, it simply gets a little weird under certain types of lighting (often seeming to do worse the more blazing sun there was).
Sometimes the 5D2 is too slow to keep up and forget the outer points, but for center only, when it can keep up I can swear even the old 5D2 is a bit more trustable than the 7D.