I'd say that A6 is probably closest match... it depends what your appetite is like and what you value [as in truly need vs. nice to have] in the car but generally the spec list in 6 is somewhat reduced in comparison to 8 and spec'd 6's are very hard to find 2nd hand for instance, e.g. no dynamic air suspension, no diesel V8's, memory seats, B&O, ACC, soft close etc.
I personally had number of A6's before and loved all of them as they are a great all-rounder's... however when I stepped into 8 it was a completely different experience
.
what I always say to my mates that keep bragging on how some car makes/models are better than the other blah blah... try and close/shut the driver's door on any car and you'll right away 'feel' the quality of the build
... and that's what A8 is all about.
Hope that helps...