6D vs D600 is an interesting one.
AF is severely, and I mean severely lacking on the 6D it would seem. That said, the WiFi is a neat trick, and I always get the feel that Canon is better at videoing.
It seems really with the 6D they've built a camera that's got all the latest gadgets and toys (apart from a swivel screen, but who needs one with the WiFi - not to mention they tend not to be on higher end cameras), but actually lacks in the actually taking a picture area.
I should clarify. I don't mean you can't take good photos with it, because you clearly can, but as competition to the D600, I feel you'll probably get better photos with the D600.
And that's the point here really. The D600, it seems, takes better photos.
Now to be fair. None of us can actually really comment massively, as neither of us have had a chance to use either of them, but comparison shots, and basic AF, it seems the D600 should win.
That's not to say that someone who buys a 6D has lost out. Because as I've said, it's still a capable camera with good photo potential...
The battle between the 7D and 6D may be interesting. The 7D is still a heavyweight crop camera, and actually, if you pay more for the 6D, the FF is the big gain. But you have a worse AF system, which frankly, baffles me.
Anyway, as with all of these discussions. Both will be more than capable of taking a good photo. It's just the Nikon, in my opinion, seems much more like a raw camera device, than the Canon. The Canon has nifty features, which if you're going to use, great.
As for either the D600 or the 6D against the 5Dmkii or the D700, the real thing to note here, is often, people don't want to buy a second hand camera. They might be the best option for FF, but still, plenty of people like owning stuff first hand for peace of mind, warranty etc...
kd