Had this same Dilemma myself, I had a 6D but loved sport/wildlife stuff. I really needed a second body so I was looking at getting a 5dmk3, or the 7d2 to come out. In the end I just saved my pennies and got the best of both worlds, the 1Dx.
Also I saw someone post somewhere that there is only 2/3 of a stop difference between the 5dmk3 and the 7dmk2, which i think is being very conservative. Unless the 5D3 is quite a bit worse then the 6D/1Dx (I haven't actually used a 5d) then i would say the difference is most in the 1-2 stops range. But I find, particularly with landscapes, it's not he difference in how high of and ISO you can shoot at on them, but rather how much noise there still is in low ISO shots in crop sensors vs full frames sensors. Shooting on an tripod I typically shoot in the 100-800 ISO range and full frames shots always look dead clean, but often the crop sensors like the 7D2 show noise in the smooth areas of the photo like the skys, and you definitely can't pull the shadows up as much.
Just a quick example, not exactly a fabulous photo but for illustration, this handheld shoot at 800 ISO on the 7D2, the dark areas have very little detail and have become very "mushy", which is something that wouldn't have happened if I had been using one of my fullframe bodies.