I keep reading statements (a ew) that say "If you don't know the reason for buying a FF camera, you don't need one.
So, in this case we have a FF vs CF inherent in this answer - so, maybe those folks who own the FF 6D will explain why they have it.
We know the 'reach' on the CF appears longer when shooting conditions are the same. So, why would one buy/use the 6D at all. Does it change between 'small birds' (reach) and FF frame for a portrait or mountain scene, for example. Or, did I just answer my own question?? Not sure, haven't shot with either one, but am contemplating one or the other soon.
Don't want to hijack this thread, but that answer seems relevant to the OP question too.