The RS4 engine is one of the best in production. 100 hp per liter. It just rips. Yes, the platform is a bit "dated", admittedly, but it is optimized. The tuning that has been done really makes the car balanced, despite the front mounted engine. It is an amazing car.

The S5 is sexy - it just is. I see one every now and again and wow. The interior is far better than the RS4. Better balanced chassis. It will be slower, but sub 5 second 0-60 is not slow.

Problem is, you can't get an S5 now. The RS4's are going out at stupid discount. Miss your opportunity now and it will be years before a car of this caliber will be going out at these prices again.