People do seem to disagree about the seats a lot between these, and from the people I have spoken to on the whole they prefer the Audi for comfort and ride.
I drove the A4 2.5 TDI down to the french alps and found it a very good drive all the way and had some fun doing the twisty road to the resort. It is a solid car.
I drove a BMW 320 D (I know smaller engine and all but regardless of power) the level of comfort was far less and I found myself having to take more breaks as the seats were disappointing to say the least. It did not have some magical set-up for cruising this distance on motorways which made it more enjoyable and neither was it any more fun on the twists and turns up the mountain.
Just my 2p. I am not a fan boy of either, but like people to try both out as everyone wants something different from there driving experience.