I would say there is marginally less noise in the 1D X, but nothing to really use as a reason for going with the 1D X and only visible side by side. However, to me, the colour reproduction is better in the 1D X, as the MkIII has a slight yellow tinge. Again though, that would be easy to correct and could be done with a preset to save time. Most probably wouldn't benefit from the 1D X, but those that need the features (e.g. weather sealing, speed etc.) would happily pay the extra. AF accuracy may be similar, but the reported increased AF/tracking speed of the 1D X could be the difference in getting the shot, which alone would be worth the extra to pros that have to rely on it to get the shot.