What's your input?
Money isn't an issue since I've been saving for some time.
If you don't know which of the two to buy then you should be neither.
Because you don't understand your photography well enough to know what you actually need.
These cameras are so different that anyone that needs a 1D4 simply wouldn't consider a 5D3.
They aren't that different, and if someone asked abouth the 5d2 and mk4, your point is a valid one. However, coming from the 1d3 and 1d4 myself and now bought a 5d3, they are different in size and weight, but the AF of the 5d3 is way better than that on the mk4. IQ is much better, and also the goodness of fullframe. And a lot of the things of the 1-series is included in the 5d; MUCH less shutter lag, MUCH less blackout time, dual card slots, zoom in directly to 100% at the focuspoint used.customization of buttons, big bright 100% VF, better battery, I get 1000 shots. Setting auto-iso range and use it in m-mode. New advanced codecs for video, audio-feedback, audio-level control. A dedicated switch between Servo and One shot, spot-af with all lenses. Setting the lower shutter speed limit (although not fully customizeable). Proper placement and size of the dof-button. And I'm sure others will think of a few other things.