I would choose the A6 over the 528. I had the A6 as a loaner for a week and I was very impressed. I am not crazy about the exterior as the styling is a bit too ordinary for me, but a very nice car. The interior is actually a tab better than the 5 series IMO. I do not thing you can really go wrong with either car, but I give the edge to the A6 here.

 

Driven the A6 3.2 and 4.2, and the 530i (05 and 06 - better engine) and the 545i and 550.

As a driving machine, the 5 is worlds better. It really is. Steers nicely and feels more agile. I personally love the styling, inside and out. The Audi 3.2 is heavy and not super responsive, and the sports suspension felt harsh. The 4.2 is a pig and not fast for a V8. The Audi interior is first rate but I thought the E60 BMW closed the gap somewhat. The new Audi interior isn't much better than my 2003 A4 (which is nice).

Don't get me wrong I like Audis. But, BMW makes a better driver and uniquely styled car (inside and out). Get the Audi if your like AWD, a nice interior, and a better I-drive interface (ableit a cluttered dash). If you want a sports sedan, the 5 is the way to go. Infiniti and Cadillac CTS would be the non-German option for sport.



CVT Benhogan

 

+1 ... this is when somebody wants you to say what is in his head

 

I've driven both and they each have plus points over the other, but on balance the 5 Series is the better car.

Audi - Cheaper, better equipped, more comfortable standard seats, more 'modern' feel, MMI tons better than iDrive

BMW - Handles better, quieter, smoother ride (sports suspension smoother than Audi standard set up), more prestigious, retains greater percentage of it's value at resale.

I thought the A6 was a good car let down by appalling suspension. My kids found the rear seat backrest to be too upright as well. Faults on the 5 Series are less serious and can be mostly avoided (e.g. standard model improves ride quality over sports models and avoiding BMW's vastly overpriced and temperamental Sat Nav, voice control etc means you don't need to use iDrive too often!)

My comments refer to the UK.