Couple of things......
1) Anyone notice the Digic 5+ in the specs?? <------ notice the "+" What's the "+" for compared to the two Digic 5's in the 1Dx?
2) No 1Dx AF and metering....said this about a dozen times in the past weeks that it would never happen. The processing power isn't here for that, 1Dx needs dedicated Digic 4 for the AF and metering. Lot of folks said it would be otherwise. Reality wins as usual.
3) Native ISO to 25,600....two stops higher than the mk2, but until we see it, that does not mean 25,600 is like 6400 on the mk2. Personally I hope it is, that might tilt me to buying one despite a few things I see I need that are in the 1Dx.
4) On the subject on high ISO IQ performance.....the mk2 goes to 25,600 native...BUT...the 1Dx goes to 51,200 native ISO. If we are to take this as any indication of performance, I had also said the 5D3 would NOT outperform the 1Dx in IQ. This hints that is the case, as I seriously doubt a camera Canon touted as a merging of the 1D and 1Ds lines but with better performance than either would hand the IQ crown to a body costing less than half the price.
5) 6 fps....not the 6.9fps/7fps we saw in the earlier specs. That one item is a near deal breaker for me and why I'm strongly leaning towards the 1Dx. I have a 5Dmk2 and a 7D, I shoot work across the spectrum....kids...high school...weddings...studio...sports. I need the higher frame rate and 8fps does that for me. I'm not looking to have "a camera for X and a camera for Y and a camera for Z". I just want one and a backup body. The 5D3 does not go far enough for me to do that if I sell the 5D2 and replace it with the 5D3. It comes close, but I'd still be shooting the 7D for the frame rate.
What could make the difference is if the AF/Metering in the 5D3 is improved enough over the 7D to compensate for a lower frame rate....in other words, I might get fewer shots in the burst, but the AF improvements result in more of them in solid focus. In addition, if the 5D3 shares the 1Dx's ability to shoot RAW's of 50-60 frames with a decent CF card before the buffer starts to complain, that could, with improved AF, offset the 6fps vs. the 7D's 8fps vs. the 1Dx's 12fps. I'd be happy if I can shoot 6fps and get 30 RAWs before the buffer causes the frame rate to stutter.
So, if the 5D3 can shoot 6fps sustained for 30 RAW shots, with AF and metering resulting in a higher number of in-focus and well exposed images, I'd give it serious consideration over a 1Dx. If it's ISO noise at at 6400 is improved by 1 to 2 stops, that will be a bonus. If it cannot perform at these levels, I'm going 1Dx to get to where I need to be.
I'd much rather spend $3500 on a 5D3 than $6800 on a 1Dx if I can get 90% of what I need and no deal breakers. My gut tells me the 5D3 will not do this, but I'll wait until it's out and in some folks hands before deciding. I have time, not like I need to buy a body today or in the next 3 months.