I can think of only three 'advantages' for the 1D X over the 5DIII for a primarily landscape use:
1) if you need to shoot landscapes in rain
2) if you plan to put a BG-E11 grip on the 5DIII and mount to a tripod (grip adds a bit of flex)
3) if you shoot handheld HDR, the 12 fps would help
Personally, if landscapes were the main thing I shot, I'd get the 5DIII.