I have a E61 (535XIT) and love it, the power and handling is great, when you sit in the drivers seat you think you are in a sedan. I would suggest getting comfort seats. I went with the standard tires and wheels to avoid run flats. The power rear hatch with privacy cover works really nice.

The MB does not have the power of the BMW and just does not handle like a BMW.

The A6 has a soft suspension and is about to be updated. Yes the 5 is also being updated but the current model is much better than the Audi.

I never drove a 530 but the 535 has tons of power and the lease and purchase deals on new cars are quite good. The 2009 has a somewhat updated I Drive. The 5 got a mid cycle update in 2008 if I was buying used I would try to get a 08.