A couple things when looking at the 6d specs:
* 1 cross focus point in the center only
* Shutter speed only up to 1/4000th may be a problem for fast prime work or large apertures in general. I would not like to have to put a fast prime lens on another body just to shoot large apertures...
* Flash sync is 1/180th which may or may not be an issue for some.
* They do specifically mention the camera focusses at -3EV, there is no mention like that for the 5dm3 that i could find but that may just be marketing hype to make it appear to stand out.
* Canon Does mention a "silent shooting mode". I don't yet know how that compares to the 5DM3
Finally canon has made a camera UHS-1 speed capable not just compatible.
* If the info is correct the images should look the same as the 5dm3 as far as iso and digic processing but given the sensor is different they will find a way to make it be dumbed down. In the early days of Canon FF the photo diode sites being much larger than the APS-c's meant more light gathering and thus cleaner better images which i think still applies given the 1dx is 16mp. Potentially if no scimping went into the sensor the 6D's images could be cleaner than the 5dm3, but that most likely will not happen.
It is funny to me the nick nacks they add to the camera. One i like is iphone remote app capability. Why in God's name didn't the 5dm3 get this feature???
* The Gps, i could care less. Some may think its cool to have no doubt. People who travel may find this feature being in camera enough reason to purchase this camera, which i guess i could see myself in that position had i traveled the world like some do.
* One thing i am happy to see is that it has the lock on the shooting style dial like the 5dm3.
* Although i prefer the Cf cards over sd by a mile they aren't so bad. If you are a wedding photographer the 6d would be your second body.