Depends on why you're looking to upgrade. If it's just you have the itch for newer tech / nicer interior / more space, then the S8 is a good choice. I owned a 2007 S6 and, although I loved every minute of it (torque aplenty and mine had a non resonated milltek exhaust, which was heaven on my ears), I wouldn't buy one again...just too nose heavy for me / didn't handle well enough. I only owned for a year, so never ran into carbon buildup issues, but it is an issue to be aware of on those engines. Also, the S8 has a lot of tech on it...I passed up buying one last December because I couldn't get comfortable with the "what could go wrong" costs of ownership and the one I was looking at had a code for the suspension that couldn't be cleared but also checked out fine when Audi inspected. Personally, I'd rather upgrade an RS6 or pay for maintenance than roll the dice on an S8...but that is me, and I've always wanted an RS6...hence why I linger on this site but don't own an Audi right now lol...
Good luck with the decision either way.