A8 v A6 comfort

[goldsax]

my 2001 4.2 over the past couple of months has needed were and tear items e.g. front suspension, front callipers, parting sensor etc......
every time i walk up to the D2 i love the way it looks and also the way it drives.
however the car is starting to feel a little old. i.e. not feeling as smooth over road surfaces etc.
i do 25k+ miles annually.
i am ‘possibly' looking for a replacement for the D2.
i know that the easiest answer to my question would be to go and drive the cars that i am interested. however i thought i would ask members first so as to narrow the search.
next car will definitely be a Quattro.
is the D4 vastly better then the D3?
is there a large difference between a A8 and A6?

thanks for any advice.
raj