If you're looking for headroom, the A6 is slightly more comfortable than its more curvaceous A7 sibling–but the A6 isn't vastly more spacious by the numbers.
Functionally, the A6 has some key differences versus the A7 in back-seat accommodations. Two six-footers can fit back there, although knees may be mashed against the front seat. On the other hand, the A7 feels a bit roomier, though both entry and headroom are tighter due to the lower roofline.
In front, the A6's seats are situated in a space with very good leg and head room, though knee space is slightly trimmed at the wide center console. Audi's seats are very comfortable for long road trips, and the headrests get a special nod--they don't stick out too far, a problem with other active-headrest designs. Front seats are heated as well, and on the most expensive models, they're ventilated for hot-weather comfort.
Trunk space is obviously smaller than in the A7 hatchback, but at 14.1 cubic feet it's even a bit small by luxury-sedan standards; the cargo floor is also quite high. All A6 and S6 sedans have fold-down rear seats, although they're not completely flat-folding.
Like many other German sedans, too, the A6 doesn't offer up tons of space to store smaller items. Cupholders are small and maintain a tight grip--which might only be to your liking if you drink mini beverages. The A6's doors do have molded-in niches for water bottles, but the armrest bin is shallow, and the glovebox runs lean.
The A6 is 193.9 inches long, right in the median of the mid-size segment, but Audi's made an effort to give the A6 a longer nose and a shorter trunk lid (in part because of its driveline orientation). That nets less space inside, though with two more inches of headroom versus the A7, the A6 is better for taller rear-seat passengers.
Overall, the A6 family of sedans has excellent fit and finish, a tight cabin feel, and an interior that blocks out road noise a bit better than the smaller A4.