I almost purchased a MY13 S5 coupe. The dealer provided me with both an S4 and S5 for an extended (24 hr) test drive. Very capable and impressive car. For me, it was comfortable. However, like blitzboy76, I just couldn't get the M3 out of my head. It was the added visceral experience with the induction and exhaust sound. Plus I preferred the seats vs. those found in the Audi. So yesterday I placed an order for a M3 coupe.
Either vehicle is a great buy but I strongly encourage you to take a extended test drive (i.e. greater than 10 miles) to determine which is better for you.