I don't think mercedes are much better really, they start with a great chassis and then seem to do everything they can to make it boring (read: safe). I quite like my audi, it's extremely refined, very quiet inside and very well put together. The ride is good, I like stiff suspension and could happily accept another 1 or 2 kg/mm stiffer springs.
Unfortunately the chassis design isn't for fun, it's geared to understeer and the computer (which you cannot turn off) won't allow confident lift off oversteer. Having the engine, gearbox and differential all over the front axle makes for a car which is difficult to turn it when travelling at speed.
The engine is by far the best diesel I have ever driven (including the current jag xf 3.0) but it's still distinctly diesel and fills you with a sense of disappointment every time you touch the throttle. lag...minor boost....dimishing power....change gear....rinse and repeat.
I do agree with you - give me a bmw any day. The audi, however, is a very nice car and suits me for the 3-4k miles a month I am currently covering.