You guys will probably think I'm weird, but depending on how improved the 70D sensor turns out, I'm thinking about selling my 6D and going to two or three crop bodies. (Downgrading from FF to crop? zomg!)
I love the sensor in the 6D, but it looks like everything else about the 70D is shaping up to be better than it's bigger FF brother. Faster flash sync, better autofocus, higher fps, articulated screen, built-in flash... I can't think of anything on the 6D that is better besides the sensor (don't use GPS). Switching back to APS-C also means cheaper bodies and lenses, which means I can afford more bodies and lenses! (f/1.8 Sigma zoom anybody?)
I would trade some sensor performance for a bit of everything else. It looks like the 70D is going to offer a lot of everything else.