Yeah, the D600 does look very tempting, that plus 2 nice lenses or 1 very nice lens would easily beat a 5D3 body-only, were I starting from scratch.

And it's not that the 6D looks bad compared to the D600, like you say most specs are still unannounced (i'd like to know if it has interchangeable focussing screens, that's another plus for the 5D2 if the 6D can't), the D600 is also pricier, but it has the specs to justify the extra $400 over the 6D.

I'm still comparing it to the 5D2 though, as Canon's 'entry level FF'.
Sure, the 5D2 may be (if not, then soon) discontinued, but there's almost nothing to count as an 'upgrade'. Or at least, 5D2 to 6D, for every upgrade there also a spec that's a downgrade. It's a replacement, surely, but it's as much of an upgrade that you get in an EFs 18-55 IS I -> II.
And if there's very little between them, they may as well be the same camera, and for that i'll take a used one over a new one.
Maybe the noise or DR turns out to be streets ahead of the 5D2, even enough to justify a few $hundred or so? Then it'll be almost into beating the 5D3 (for IQ), but I just don't see canon doing that.