The cars live in immensely different worlds.
One is strictly a sports car (C6) and serves it's function very well.
The S5 is an all around great car that is a great in any climate, temperature, and exudes sportiness and class.
If you want something to enjoy everyday regardless of weather, keep relatively stock, and look like a million dollars, while driving around in an interior second to none, S5 all the way.
If you want a car to make quick and take the the dragstrip on nice days, C6 without a doubt.