Well, i strongly believe that finding the proper camera involves an irrational impulse, like falling in love. When i got my first DSLR, i was sure i would have got a Nikon D3100. In the shop, right next to the D3100, there was this Canon 550D, priced about the same. I took it and, BAM! love at first sight. It just felt "right": it fitted my hands better than the Nikon, i liked the viewfinder more, and the buttons were right where i would have put them if i had to design a camera.
What's the point of this? I think that ease of use is as important as technical specifications. You like your 6D and you're happy with it, and it looks like your wife, which is going to be the one to use the new camera very often, will like the 70D. So 70D seems the most sensible choice (and it's the cheapest of the 3 as well!
). Have her try both the 70D and the 6D and tell you which one she likes more. If she picks the 6D, then you have a justification to get yourself a bigger toy.