It depends what someone prefers. Seeing the interiors IRL helps a lot by making an opinion.

For me E-class interior is a typical "Neo-Baroque" style: shiny, kitschy, curvy. I can imagine 65-year old Hans and his 60-year old wife Helga in that car, driving to the opera. For old peopl's taste, or the younger who prefer rich conservative and over-decorated pure luxury design.

A6 interior is a (starnge?) mix of old & new. Kind a "Art Nouveau" style. Combining some elements of 90's interior design with a new technology, and from the tech derived design elements. It's the safest way, the most evolutional, simple, stylish - for medicore people, and for people who do not want rich design: classic neo-baroque of MB, or modern avantgarde one of BMW.

5er interior is pure modern avantgarde - kind a "Deconstruction" style. Complex design, many different lines & surfaces, very dynamic, over-styled. Not for everyone. Mostly for young (at heart) & dynamic people who like to experiment & do not affraid to be different. And for those design enthusiasts, who like the fresh ideas.

We have 3 very different approaches: classic (MB), neo-classic (Audi), modern (BMW). Very good for the consumers. A lot of choices.

For me? 5er of course.