So, I have a 6D and a D600 in hand right now...

Another thing worth mentioning: I briefly revisited the idea of the continuous AF capabilities of these cameras. Instead of trying to track a living mammal in a poorly lit room, I tried something a little more static. Placing a coffee mug on my desk, I re-tried the continuous AF modes by panning the camera back and forth on a tripod.

I won't bore you with photos (though I will, if asked), the Nikon D600 did better here - especially in '3D mode' - because the outer AF points are grouped closer together than the Canon 6D's. There's a significant gap between the center 9 and the outer two that causes it to lose focus during slow panning. The D600 tracked the subject well, so long as I didn't pan too fast, there was enough contrast, and the initial point of focus didn't wander outside the AF area.

Might I have better results with a different lens? The answer is, "Probably." Would I consider these cameras to be capable of tracking continuous movement? Probably not. I still maintain that the AF area doesn't cover enough of the frame for that. There are better cameras for action photography out there.

Cheers

Michael