I've just "upgraded" my 156 JTS Lusso to a 159 3.2 Q4. What a car!!!
The 159 is obviously a replacement for the 156, but really driving the 2 of them you wouldn't know it. The 159 is so much better in every respect.
One thing that's ****ing me off and I haven't got a solution yet is how to get my DAB radio installed and working with the steering wheel buttons on the 159. Also wondering how best to add Bluetooth to this setup. Really wanted Blue&Me but since I bought a new one "off the shelf" rather than ordering it via the dealer I had to take the car that was available rather than the exact car I wanted.