EDIT: holy thread revival + you bought one, so this is largely irrelevant now. In my defence i opened an old laptop that i've clearly not used for a month and had this thread already open.
I'm biased, but i'd go out and buy a C63 again tomorrow if i had more money lying around. I was very close to buying a black series.
Audi are fine, i had one on order. I've driven both models you mentioned:
Audi is a fast estate that has something special about it.
Merc is a batshit engine that happens to have an estate body on it.
Both have terrible fuel economy, but are both great cars. As someone else has said - i planned on owning the C63 for <1year, ended up having it nearly 2 - it's gone for two reasons:
1. two f**kwits crashed in to each other and then both slid in to stationary me...
2. I live in London and can't justify having two cars with a combined 12.2 litre capacity and 18 cylinders.