Before I forget to recommend it -- RENT BOTH and use them in a particular/demanding shooting style that you know well.
I won't break down the features for you so much as there are two considerations that automatically will have you select the 5D3 -- the video quality and the AF.
For video, I don't shoot it, but I'm told by many on this forum that it's a night and day comparison with the 5D3 outclassing the 6D.
The AF (though not the metering) is the 1DX's system -- it really is world class. If you regularly shoot moving targets*, the 5D3 is the clear choice despite the cost increase.
- *An exception would be if you shoot moving targets at the long end. Many threads on this forum have run the financial numbers on this. If you are regularly shooting moving targets at, say, 400mm or higher, I'd recommend you stay with crop so you don't feel compelled to either (a) buy the crazy pricey long glass, (b) learn to love constant teleconverter use, or (c) enjoy F/4 and F/5.6 long lenses. All are viable -- I am not picking on T/Cs or slower lenses -- but having to use them may have you questioning why you ponied up so much for such a great body. If you are this person, don't leave crop; I'd wait for the 7D2.
If you do make the plunge, don't be discouraged that the 5D3 is complicated. I made the move from a crop to a 5D3 and it was fairly straightforward over time. The camera is powerful, but so is the interface. You can dumb down the AF to something simpler if that suits you (I actually do some zone focusing rather than cluster or point in some applications), but you still have that killer flexibility and tune-ability if you want the Ferrari-customization for tough servo tasks.
All that said about the 5D3, the 6D does make more sense if:
- You shoot a lot of Zeiss manual focus lenses ( sorry, had to)
- You shoot a lot of LiveView
- You shoot a lot of landscapes
In those cases, a super-flexible AF system might go to waste there.
Also, if you are a heavy social media guy (who can live with onboard processing), the built in WiFi and GPS are a nice value and nicer convenience.
Just my two bits. The 6D is no slouch, but it lacks some non-trivial things a veteran shooter might need. You have to decide what's best for you. Again -- RENT BOTH.