I think the camera that Canon really jumped over Nikon with was the 5D3, when pitted against the D800 most photographers will find the 5D3 more useable. Sure, the D800 has it's uses, but for a more narrow segment of the photographers out there. The 1Dx vs. D4 is probably a close race over all, feature for feature.
So that leaves the 6D vs. D600, and I'd say the D600 really compares more to the 5D3 feature for feature, and when you factor in the price, may be a better buy. So in the end, over all Nikon may still have a better lineup of full-frame cameras to appeal to the widest group of photographers. Canon has limited the 6D so much that it's opened the door for Nikon to market the D600 to photographers who are looking at choosing between the 6D & 5D3, they can get the D600 and not have to choose, effectively. Sure, the 5D3 is built better and probably performs better over all, but when it comes to marketing to the widest possible number of photographers, the fact that the D600 has most of the key features in common at a much lower price tag means it will have a wider target market.
I'm really not sure anyone can make a call with 6d vs d600 at the moment. The 6d is still really unseen and untested. So m,any if's with the 6d, and from what I see with the d600 --- those that are commenting are saying that there is no point in having so many AF points that are crammed into such a small section of the field of view. If the 6d AF is spread more evenly, even with less points it may in fact hold its own. If the ISO in the 6d can come close to what the mk3 can do between 3200-12800, then well the game is really on (d600 native iso is 6400). If's and if's and if's though, until the 6d is out there it's hard to make any call of winner or loser.