Canon has some catching up to do with respect to sensor performance as measured by http://www.DxOMark.com. Canon doesn't even come close to the top performing Nikons. (High score is better.):
96 Nikon D800E
95 Nikon D800
94 Nikon D600
81 Canon 5D III
79 Canon 5D II
(The Canon 1Dx is not yet rated.)
What are the chances that one of the reasons for the new sensor in the 6D is to catapult Canon's sensor performance into the mid 90's? I can't see Canon doing that considering the $3,500 EOS 5D III just came out and has a score of just 81. But Nikon's new $2,100 D600 kicks butt with a score of 94!
Sensor performance isn't everything... but, if I were to choose Nikon or Canon today, I wouldn't be choosing Canon.
1. their overall scores are weird, how do you combine so many different performance factors into one number that would work for everyone? you can't
2. their PRINT generally seem to be pretty good and quite useful to compare various aspects of one camera to another though
3. their lens tests, unlike their sensor tests, appear to be a mess, i don't trust them at all
4. I doubt the 6D will make a big leap, you do much better at high iso for SNR than the 1DX and I'd doubt, just a couple months later, they introduce a new sensor that has better DR at high iso than the 1DX or than both at low ISO, that would be hideous planning for the 5D3/1DX. Then again, it would be nice in that at least it would prove they can do it. It is odd they have a supposedly new 20MP sensor.