Usually this query garners fervent rhetoric from those faithfully wedded to Canon.
I actually divorced my 5d2 and hooked up with some D800s and plenty of other Nikon gear specifically for landscape purposes, tho not the 24-70mm.
I had a very early production 5d2 and it was very noisy, complete with perceptible vertical banding in midtones like clear blue sky and it got worse as you went into the shadows. Metering was randomly off, but it behaved consistently in full manual. That camera was certainly not up to spec for serious landscape work and was a very disappointing experience for me after all the positive ones I had with my earlier Canon bodies.
Nikons with Sony sensors eliminated banding noise problems completely from my workflow, what a pleasure!
As some state, the DR difference between canon and d8x0 is not that big a deal, and for the most part that's true, altho I'll take all the DR I can get.
What's related to DR, however, but not yet mentioned in this thread, is the Fixed Pattern Noise issues (vertical stripes) that Canon is only recently making incremental improvements on. The 5d3 is not significantly better than the 5d2 in this regard. The 70D is probably the best crop body from Canon in this regard but still no match for the cleaner files you'll get from any Sony Exmor equipped body (nikon, pentax, Fuji?)
Now I likely had a lemon of a 5d2 but the early production 5d3 I tested still wasn't much better. If you need the best low ISO performance, get an ABC camera (anybody but canon).
Summary; if you currently shoot Canon for landscape, and you are satisfied with the results, then a D810 is not likely to impress you all that much more. If you need to heavily process your raw files, the D8x0 will provide considerably more leeway to do so, especially in the shadows. The extra MP of the D800 may be useful if you're printing larger than 36" wide but you do need good glass and technique to get the best results from it.
As for Nikon's 14-24mm lens, I use it. It's very nice, but it's far from perfect. It's fun to use, provides fantastic ultrawide images but can have significant CA in the corners. But, so far there's really nothing else like it.
An effective alternative is Samyang's 14mm prime + Canon's new 16-35 f/4L combo.
Edit: condition altered on leading statement.