Another A6 or an A8?

[as400]

Will be starting a 150 mile perday round trip commute soon.

Have had 2 Audi A6 3.0tdi before (A6-C6 Le mans) had no issues with them and miss them.

Am in two minds now between A6 (C7) or maybe an A8 (D4) both 3.0tdi.

Each 75 mile journey will be 90% motorway M27/M3 probably outside lane steady 80mph.

Will I see 40mpg from the A8?....take it they all have air suspension which will be a new thing to me...is it noticeably a nicer ride than traditional A6 springs?

Same engine but is the A8 any likely to be more unreliable?

I tend to buy 1 owner from new FSH only.

Anything in particular on the A8 I should look out for?

Thanks for any advise!