I owned a 2007 S6 so can speak from experience. It's not as fast, not as mod-able (really not much you can do, outside of a tune that is overpriced and eeks out a few more HP/TQ, an exhaust, air filters and sway bars) and not 11 years old - so it's likely going to be more wallet friendly on repairs and it doesn't have the "major" issues the RS6 can have. However, I loved my S6. I had a non resonated milltek exhaust and between the sound and the always on torque of the car, it was a blast to drive. If you buy one, put on the best summer rubber you can afford, because the car is massively nose heavy and it feels like it in the turns. I would have eventually upgraded sway bars, if I had kept it. As for the comfort/tech, it was great.
You will need to do a carbon cleaning at some point...pretty common maintenance item on the car. You also have 10 cylinders to maintain...I believe the 55k mile service would have been around $850 at the dealer, maybe more (cheap for the guys on this forum, I suppose)...I was able to do it for half that through a friend/audi tech. I never had to do brakes or tires in my ownership, and other than a seat belt sensor in the back seat that went bad, I had no issues in my ownership - just gas and oil changes (had it for 2 years).
If I was buying now, though, I would go RS6. I've always wanted one, hence my lurking on this site...but I went 2007 S6 to 2002 S6 and now, to quiet the demons of my car buying disease, I'm leasing a 2014 Infiniti Q50S through December 2014...then, I hope...it's into a used S6 or S7...