Originally Posted by Uiop
They are both cars with different characters. RWD v quattro, 4.0 v 4.2, 414 v 444, 295 v 317. Just pick the car that you are attracted to. This is why I picked the RS4, sure it has maintenance issues, but if you want to maintain a sports sedan/coupe, a carbon clean for a few hundred wont kill you.
The weight difference taken in consideration, this shows more the similarity in their character then the difference, you also forgot that they both have 7speed dual clutch gear box!
A C63(big displacement, lots of torque) would have a different character but RS5 and M3, prety similar(small displacement, low torque, high reving)!