Damn it. 70D it is then.
Here is another 'nonsense comment' that may or may not help to those who decide on crop-to-FF switch:
It was easier for me to buy 6D as 400D replacement =
1) I thought about replacing 400D for last 5 years with a camera that either has better ISO/indor-focusing or a camera that is smaller and lighter than 400D. So, during this year I kept 3 cameras in my list of possible candidates: 6D, 100D, EOS M, Powershot G1 X.
I shoot mostly two types of pictures: printing equipment (like Heidelberg or Xerox presses) in printing factories or at exhibitions (for my magazine review job) and architecture/landscapes (while traveling/vacations). I do not shoot birds, football players, other types of animals, planes, fast moving cars, moon, etc. unless I have no other choice. (Maybe some fish and turtles with G7 in water-box...)
2) About two months ago my wife complained that our small G7 'is not wide enough' for her to shoot European cathedrals (while she travels by herself with our small camera). I was overjoyed by such a comment and used it as excuse to buy EOS M two-lens-kit+11-22 as G7 replacement.
3) EOS M is a great camera! Cheap, small and light (both body and lenses). Arguably: "the best amateur camera Canon ever made".
It has cool touch screen: you can compose a picture, than tap a finger in the area you want it to focus and press down the shutter button. Done! No more usual "zoom-focus-recompose-then-zoom-focus-recompose-again".
4) But EOS M's APS-C sensor has the same limitation as other cropped-sensor cameras: pictures at ISO 1600 (and above) are close to unusable because of too high noise level (and not very high dynamic range).
5) After getting EOS M as a replacement for G7, to replace 400D with any other Canon's cropped camera made no sense. 6D was the next logical choice of 'equivalent type of replacement'. And, yes, I'm happy that I made it. Now I can be busy to relearn how to use the lenses I already have, what settings to use now and then. Great!
6) We also plan to spend next New Year's vacation in Iceland (with 4 hours of dim light at day and possible northern lights at night). 6D looks much more like a capable camera for such conditions than 400D, EOS M and the rest of the cropped crowd...
Edit: changed the first sentence wording to make the rest of the post more relevant