The Audi does nothing for me at all, I'm sure its a great car but just this is what I feel when I see one
I love the character of the Mustang and it has bags of it, plus it's excellent value for money. That being said, there has to be a reason for that and I'm guessing (because I don't know) that the quality of finish perhaps does not match its rivals. I don't think I could live with that personally, not when I'm spending £35k.
The C63 I think does have some character to it, the 4 litre engine in it is terrific and I like the "One man, one engine" ethos of AMG. All I can go by is my own experience I guess and I've been very impressed by my C55. Its a beguiling drive and the V8 burble / growl is terrific when on song and the engine in mine isn't even supercharged.
I guess the usual applies, you need to drive them to really know which one you want. Put it this way, if you were in a room with all of them right now and you had the money in your hand - you would know within moments which one you'd be leaving in.
For me, for the quality, I would go C63. If I needed another car with character and a bit of show / a laugh, I'd consider the Mustang....then probably buy the Merc anyway