The 7d is notorious for the imprecise af system, but op is talking about a 5 year newer *70d* (read the op).
It's the same focus system, just a bit neutered. Oh, and I agree with you concerning the consistency. Pretty terrible at telephoto.
Did you read the 7d manual if that's what you get :-p? Of course I believe you, but generalizing that the 6d gives more keepers for action shots is really stretching it, I suspect even most 6d enthusiasts would disagree here. Given that the 70d has nearly double the fps of the 6d, even *if* some more shots are out of focus (which I somehow doubt) you'll still end up with more keepers.
I've used both cameras in the field for ursine and ungulates, side by side over thousands of images. The 6D is the better camera for everything. The 7D focus system only seems to do well with high contrast subjects like brightly colored sports jerseys at telephoto. There's something about it having to grab fur and feather that doesn't work all that well, at least on my copies.
Now, the 7D does pretty well shooting high contrast objects (IE not fur and feather) in daytime conditions, but then again, what camera doesn't do well in these conditions? If someone wants to shoot sports or racing from 9 am to 4 pm, great camera. But for dawn and dusk fur and feather, the 6D embarrasses the 7D. The 70D, with my trial runs, felt similar, and I wasn't surprised to learn it has the 7D's focus system. Which means I won't be buying it, which is a shame.
True, 70d "only" works up to -0.5lv - but did you calculate how little light this already is? In these conditions, I wouldn't be tempted to continue shooting with a crop sensor anyway (ymmv).
The camera last to come out at dawn, and first to be bagged at dusk is probably not the better camera within the context of wildlife photography.
The 70d is not a low light camera, which definitely limits the op's requirements for dawn/dusk shots - the very reason I bought a 6d. My advice is just not to underestimate the 6d's limits concerning video and tracking, you get what you pay for - the 6d is not a "mini 5d3". The question is what corners the op wants to cut - though since he didn't reply since then he won't read these replies anyway.
I agree that the 6D is not as responsive as the 70D and 7D. But what it lacks in responsiveness, it makes up for in sheer IQ, dusk/dawn usage, fur and feather, and IMHO, a higher keeper rate even in action shots during critical wildlife times.
I didn't want to think this was true, trust me. How could a lowly, slow, cut down bottom-end Canon FF outperform the top tier crop bodies? I prefer the build of the 7D. I prefer the flip out screen of the 70D, FPS, and buffer. But in the end, when I went through gigs of images, it was very clear, even on similar action subjects, which the better camera was.