I just bought a 6D after seeing the increased dynamic range and iso capabilities it offers over my 7D. Having said that I must say also that I love my 7D. I've owned the original Rebel, xTi, 40-, and 50D bodies, and the 7D is a radical upgrade to the best of those bodies on their best days doing whatever shooting they did best. I've owned this camera for some 3 years and it has been a fabulous tool for capturing the images I wanted, within its own limitation, which is its high-iso noise handling - for me, anything over 1600. I generally expect to get good shots to that point. I shot a Bulls game with a 70-200 f/2,8L IS at 2000 and the majority of the shots were very usable. Up to 2500 is usually usable for me after some software tinkering. I haven't felt comfortable beyond that, though I have even seen other shooters get great results at higher iso settings.
The af is spot on and fast - and the 19 selectable af points are something I miss on the 6D. I have rarely missed a shot using af. Until last week my kit consisted of a 50- and 7D bodies, but I will now sell the 50D, grip and all, and work with the 7- and 6D bodies. I am still hoping for enough improvement in the 7D II to warrant purchasing it, but I am very happy with my current combination. The 6D will be my portrait, landscape and low-light tool, but the 7D out-runs it for sports and nature (wild animals), the frame rate and crop fov lends itself to that type of shooting.
As for what to do, that is not so easy to answer. If you decide to buy the current 7D, I would buy it new. The price continues to stagger downward, and it will have a longer useful life if bought new. On the other hand, the 7D II is expected by the end of the year and should be as spectacular as the original when it was introduced. I would guess that gps and wifi will be built-in like in the 6D - by the way, those two features are fabulous, even though they do eat battery power. I would hope that the iso noise would be greatly improved with the new (?) sensor and mfg process. These things layered on top of the current specs would make the II very compelling for me next Spring. If you expect to need the frame rate and fov, along with the superior af system the 7D stands out, and most likely the 7D II even moreso. On the other hand, the 6D offers clear iq superiority, and its ff sensor will probably beat out the 7D II if it continues as a 1.6 crop. The much-maligned af in the 6D requires a little more deliberation, but definitely produces; although tracking fast-moving targets may not be all that easy. The thing is, if you decide to go crop, it probably makes more sense to wait for the new technology of the 7D II.