Upload & Sell: Off
I mostly shoot musicians and dancers--NYC nightlife kind of stuff. I sold my 5Dmkii and a couple things I wasn't using to get the 6D, primarily for the low-light AF and secondarily for the "silent" mode and the slightly better high ISO performance.
The AF center point is *much* better in low light--I'd say the AF is close to two stops more sensitive than that of the mkii, so that focus-lock is relatively reliable even in ISO12800 situations. It also seems to be more accurate generally under all conditions. (Weirdly, however, it subjectively seems to be a *hair* slower at focus lock than the mkii under normal lighting, almost as if there were some extra step involved.)
The additional low-light ISO performance is better than the mkii. When I look at the RAW comparisons at dpreview etc. I estimate that it's about a half-step better than the mkii at ISO6400 and 12800 at detail resolution in the highlight to middle range. However the improvement in shadows appears to be greater than 1/2 stop, and subjectively my ISO6400 and 12800 shots are more improved over the mkii than I expected.
Shutter lag is improved and the silent mode, while not silent, is much quieter than the mkii.
The wifi is good for popping up something to Facebook, but note that it downconverts to an "appropriate" size and quality jpeg that I find to be really muddy compared to my standard Lightroom upload settings for FB (max quality jpeg at 1600xXXXX pixels).
Just to be clear, I think the 5Dmkii is an amazing bargain right now, especially used. For general purposes I think it's fantastic.
However, for my specific purposes the improvements over the 5dmkII are well worth the $700 I spent to upgrade, I'm very satisfied.