The difference between the 1d X and 5d 3 is much bigger than people realize from reading about them. The AF and the 12 fps gets you so many fantsatic images you couldn't have gotten without it. The exposure is so dead on it's ridicolous!! I hardly adjust anything in Lr.
The tracking is awesomely superior. To me it's at least a stop better noise-wise, and the image can be pushed more without adding the crazy noise. The lower iso's are the cleanest I have seen, and also better than the 5d3.
Swap it vertically and have the exact same layout of buttons and the same grip is not to be underestimated.
And all of the small but very important custom functions like limiting your shutterspeed range in Av mode etc makes it way worth it to me.
The 5d3 isn't the same with AF and tracking, it isn't the same IQ wise as the 1d X has much less noise and have WAY sharper files.
It is quite obvious. Things have to be like that. Otherwise you'd be wrong to spend such an awful lot of money on a PJ workhorse. I am glad for every pro who is equipped with excellent gear via the 1DX. As an amateur who hardly commercialzes his photography but who appreciates some better high ISO and IQ, I will go 5D3 within the next months from my current 30D which I am shooting since five years. It always delivered decent results even at ISO 3200 b/w, exposed to the right. So Robert Frank high ISO shots, here I come ;-) I don't worry about decent noise nor blown highlights by pushing it to the limit since I've seen his photography. Sometimes even slight motion blur adds to a b/w photograph.
@Viggo: Did you take any pictures at "insane ISOs" as high as 102 and/or 204 k...? Would be intresting if you had some shots to share. Keep the good work up! Cheers, Peter