It depends on what your requirements are.
I did this analysis yesterday and now i updated it after reading up all the details that i could find.
I am looking between 1DX and 5D mark III.
So looking to see what are the differences between the two. The ones i can see are
1. Single Digic 5+. This ( and the higher MP) limits the processing speed to 6FPS. A slight advantage to 1DX for my needs. Would have preferred atleast 8 FP as i take Dance photos.
2. No Digic 4/iFCL metering with 63-zone dual-layer sensor. It means the same metering as in 7D - Two year old technology. To me this is a big step down from what is promised for 1DX. This means that there is not the same metering linked to the focus point as the 1DX. 5D Mark III has the face recognition according to one preview.
The auto ISO changes are also welcome. I understand that they are not as robust as in the Nikons, but i am not looking to change systems. So didnt factor into my decision.
3. AF - Most of the functionality seems to be the same. The hardware and algorithm are said to be the same.
I am also assuming this means that the ability to focus in -2EV is not there in the 5D Mark III. The difference i see is that i dont think the integration with the exposure component is to the same level. I also do not think it has the focus point based spot metering that was promised for 1DX.
4. CF /SD card slots vs the Dual CF card slots. I dont have many SD cards and the one i have is a simple 1 GB card. to me it would be better to have CF cards only, but then the SD card gives you networking capability through the Wifi cards. Have to think more about what i need.
5. HD Movie. From what i learnt from yesterday since i posted the previous version of this analysis, I do understand why there might be less moire for video applications. The 3x3 scaling will help get cleaner video in my understanding. So this is better for 5D Mark III. The audio out also is better than the 1DX. I think 1DX is video added to a still camera from design choices perspective, while 5D Mark III has more design choices in its design to help with video as well.
Movie compression schemes seem to be the same between 5D Mark III and 1DX.
6. HDR Mode. The 1DX initial specs do not talk about HDR mode. So not sure if this is a real difference or just a marketing spiel of the multi exposure setups in 1DX. From what ever i could read the multi exposure choices on 5D Mark III are the same as 1DX and HDR combination is new in 5D Mark III. I think this is most probably considered as an amateur function and so was not included in 1DX. I assume that 1DX users would prefer to have more control over the HDR and so in camera HDR was not included. Not a big deal for me right now, but it might be easier to get better photos in some situations easily.
7. Weather sealing - The Canon europe pro site mentions that the sealing is the same level as the Film based 1N. So the weather sealing is around 10 year old levels. But at one time considered 1 Series level! So it obviously less than 1DX. I dont need that high level. For me it was icing on the cake.
8. Ergonomics - This actually something i am worried about. My camera for the last six years had an L plate stuck on it always that connects to my Arca Swiss ball head. In my shooting i alternate between the tripod and hand held a lot. So on a new 1DX i most probably will keep the L plate also. So i am thinking that for that purpose the other handling side of the 1DX actually might be a distraction and not something i would want. Because now my hand will have to go around the L plate also and so will nto be as comfortable a handle as the regular one is. On the other hand (Pun intended) the way i shoot now with my 5D with my hand on the top of the portrait shots makes my hand ache a bit by the end of the three to four hour shooting session. This is a situation i did not think about until recently. Dont know which side to go for this issue.
I decided to stick with 5DIII for now for this issue. Just need to do more work in the Gym for it.
9. MP difference - too small to really matter to me. Coming from 12 MP (of the original 5D), both are a significant bump. Will satisfy me i think.
10. ISO difference - If the native is 1 stop lower on 5D Mark III, that means most probably the usable ISO is ISO 1600 and can go to ISO 3200/6400 in tough situations. For me coming from 5D, this is atleast 2 stop increase already for Mark III. And for the dance concerts i do take it is sufficient. There is enough lighting on the stage for me to be happy with a 2 stop increase. So professionally i am good. Personally higher ISO will allow me to take photos of parties etc without the flash and capture more of the ambience. So 1DX has an advantage - tiny bit only.
11. Silent and Low vibration modes - Not sure if i need them. Important for video. 1DX has the still photo sound reductions but it does not have the capacitive touch strip on the back wheel to use during video recording. So I think this is another area that the 5D Mark III looks more like that video was an integral part of the design decisions.
12. File formats, length etc - no difference.
13. Intelligent viewfinder - It is the same as in the 1DX
14. Shutter actuations - Not going to hit the limit in any way. My current 5D i have done maybe around 40,000 actuations. So not really an issue for me.
15. Accessories - Same as 1DX. I am not sure i am interested in the GPS accessory. I would have liked it in my recent vacation, but then my iphone had it and made do with it.
16. Networking - USB only for 5D Mark III vs the wired ethernet port for 1DX. I think with the larger files and the associated larger card sizes i would need, faster transmission is a must. 1DX is winning here.
I made my decision in the morning and put my preorder in.
Only things i would have loved to have: the metering of 1DX, and the better low light noise levels. But just those two were not enough of a reason to spend an additional $3,000.