If somebody were to ask 'I'm primarily interested in landscape photography, I don't have a DSLR preference yet and I'm looking at buying a full frame camera, should I consider Nikon or Canon?' My answer would be 100%, without a doubt: look at the Nikon D600 or D800 depending on what you want to spend. For the landscape photographer you're primarily concerned with dynamic range, ISO noise, sharpness and the ability to crop, by every one of those metrics the D600/D800 beats the 6D and 5DIII handily. And if you're looking in the ~$2000 range the D600 beats the 6D in several other areas (on paper), the focusing system for instance.
This assumes most other things are even like ergonomics, lens selection, accessories etc which may or may not be the case for you. For instance, I personally feel Canon has some better lenses available including all their tilt-shift lenses, especially the 24L TS-E which is a fantastic landscape lens and the 135L f2 (which I also like for landscapes). And there is the built-in GPS functionality of the 6D...
Now, you happen to already have a collection of Canon lenses and accessories so you have a higher switching cost and its possible Canon may come out with something that competes better with the D800/D600. However this seems highly unlikely since the 6D and 5DIII are the direct competitors of the D600 and D800 so the idea of Canon coming out with something like a 6DII and 5DIV soon just doesn't make any sense, these are the bodies we'll have to work with for at least 2 years if not more. Its more likely canon would come out with a 1D series body but thats not part of this discussion.
So, if you like trying (and buying) new things and have the $$$ to do it, then do it, I know you're considering buying a new body as it is and probably a FF one at that so you're already going to be spending money. If you sell all your Canon gear and re-buy what you want in Nikon (used) you won't be out too much more than buying into a Canon FF system, we're talking < $500 here. And before anybody bags on me for saying $500 'isn't that much', I'm talking about when compared to the total price of either kit, not in a vacuum.