I would take clean high iso to get my shutterspeed fast enough over twice the mp any day, and that is just common sense;
If your image is fantastically high res, you will see every bit of that juuuust to long shutter speed you used, and you'll have a useless image. I can tell you, going from mk3 to mk4 with the same size sensor but 60% increase in res, I had to double my shutterspeed, giving me the exact same noise under the same exact light. Now, the pictures that did stick with the mk4 had much better detail, but I had a lot of shots I kept, but can't use for anything due to (unexpected) motion-blur.
The 1d X will be the ultimate camera for me, no matter what I am shooting, and I can shoot things I could never dream of with the mk4. After trying this new AF, I had chills for the first time using a camera. It is just astonishing...
But if you do landscape, why would you buy a Worldclass AF-system and 12 fps? Buy a 5d2 for your landscapes, very little will surpass it.