With all due respect, the Nikon D810 is what you should be considering. I don't see too many people thinking the Canon 5Diii is the king of the DSLR for landscape use. I shot with a D700 for a while and made do, and the difference between it and the 800E I shoot with now is marked, obvious, and substantial; but I should note I took the time to hone both my shot discipline as well as my lens collection. There is no Canon body available now that I would consider for landscape work personally. The Canon fans may scream fanboy, so before they do I should say that if I shot more wedding/event than I do, I'd likely have a pair of Canon 1DX around my neck instead of Nikons. Different tools for different jobs.
Obviously the 50mp Canon bodies might change the equation, although I don't think the visible (in print) difference between 36mp and 50mp will be anywhere as substantial as the difference from going from 12 to 36mp, and the DR aspects of the new Canon are unknown. There is a point IMO where enough resolution is "enough resolution" dependent on each photographers expected output size.
In general if one already has a reasonable glass collection in one brand that it's not generally advisable to switch systems unless there is a real, distinct, obvious need to - not an emotional reaction, not a reason based upon buying the latest so you can be the cool kid in the forums, and this also includes making sure you've worked through your own limitations in other things as well - spending the money on training if your post processing skills are lacking, a better tripod/ballhead, etc. The reality is that most DSLRs today are very competent. As for your original question, a D750 and a Canon 5Diii would be similar except for the greater DR of the Nikon body, which IMO puts them (currently) at the top of the class for landscape work.
Note also that the Sony folks will have a valid argument for the 36mp Sony mirrorless. I don't like composing on a computer screen and personally can't stand EVF, but the Sony gear has to be taken seriously if you are considering changing systems as well.
You might want to wait and see what the new Canon 50mp monster looks like before pulling the trigger anyway.... it might (or might not) be the "one" for you