So how significant is the difference in dynamic range between the 70D and 6D. And for that matter, between the 6D and the Nikon/Sony? I'm trying to make the same decision as the OP but I find one serious limitation I face is trying to photograph choral events where the performers are in black on a platform with decent lighting, but backed by a white sound board that is well lit. Flash is not an option.
To be more clear, my situation is like having a bright hair light on the upper half of the head while the lower half is only receiving average or less light.
Well, I'm not sure what else you photograph but for the one you mentioned I'd definitely pick the 6D.