JesseC43,
The A8 will be more expensive to maintain.
Totally different vehicles, it would be more fair to compare the A8 with an S-class say an S430. The E320 should cost about the same as your C43 to maintain if not less. By the way is this an authorized dealer where you saw the cars? If you liked the A8, it might be worth looking a bit more at the Audi dealers in your area, I am sure they will have some just coming back from lease.
Good luck,
Joe Brasileiro