Hmm, 5Dc and 50D are pretty close in general IQ but at 1600ISO the 5Dc will have a slight edge, while you being used to the 5DIII now would mean the 5Dc would remain as an absolute backup as it'd be far slower in response and it'd possibly be 'uncomfortable' to switch to it.
I'm in a similar situation, don't quite need a backup yet but it would really help in some situations as my current is a 5DII and a backup of a Rebel T1i that I don't even touch, just purely an in case device. I could grab a used 60D for real cheap now, but the lack of AFMA bothers me as well, but for video work that isn't an issue, and I could limit the use of UWAs on that. While considering the 60Ds sensor isn't stellar with it's age and being behind the 5DII, I'm waiting to see what this 7DII/70D will turn out to be, if it's good and not insanely priced then I can go for that, if not then either way the 60D should hopefully drop in value some more and become a real bargain. I know other wedding shooters carrying the 60D or even 50D as second bodies so really either of those are perfectly usable.