I'm still waiting for the official announcement, and clarity on some on my assumptions above (flash sync, AF point-linked spot metering, customization). The other question is low-light AF sensitivity - all the non 1-series bodies are -0.5 EV, and the 1D X is -2 EV, so where does the 5DIII sit? I was shooting in an aquarium last weekend, and the 5DII AF was really struggling with the light level (center point, which is the best one, and 35mm f/1.4L so the lens wasn't limiting).

Right now, I'm leaning toward the 1D X. 2x the frame rate and 1 stop more ISO are part of it, but the main thing is the build quality. It's great that the 5DIII battery grip will (apparently) have the 2nd joystick for ergonomics similar to the 1D X. But, I've never been thrilled with the flex between body and grip - the benefits of the grip do outweigh the annoyance, but I'd prefer it integrated. More important is the weather sealing. Earlier this evening, my daughters were out playing in the yard in what's most likely the last snow we'll see in New England this year. They were having a blast, but I have a strong feeling that it would require 1-series sealing to withstand the below-freezing temps with freezing rain/snow mix that was coming down, -2 EV sensitivity for the AF to have a chance well past sundown under overcast, and the best ISO performance possible - which adds up to the 1D X.