That thread was indeed a blast from the past.
Having now owned the "new" S6 for over a year now, it is easier to compare it to my old RS6. In most ways the two cars are quite similar, so the differences I'll note should be taken to highlight the differences at the margin. The S6 may not be quite as fast or visceral -- it of course lacks the RS6's turbo punch -- and it isn't as special as an RS6
, but in other ways the S6 is superior.
S6 has immediate throttle response (FSI and no turbos). My old RS6 had minimal turbo lag, but by comparison it seems glacial in reacting to acceleration input out of a turn or off the line. It may not make a real difference in track times, but the experience is very different. Once its turbos spool up and the revs climb, though, the RS6 has more kick. S6 power band feels much more linear and pulls right out of the gate and keeps on pulling. The steering on the S6 seems more precise, too, though on both cars steering feel and control are excellent.
The S6 also has a better transmission (one more gear, faster and much smoother shifts), has marginally better fuel economy under all types of driving (5% or so), and has a more modern interior and controls (MMI, nav, etc). It just feels more polished. Plus, the S6 has been completely reliable, while the RS6 can require more attention for maintenance and repairs because of its more specialized mechanicals and the greater stresses they are under (turbos, DRC and all). For daily driving, the S6 is more comfortable (in the U.S. the RS6 seats were quite poor; S6 seats rock).
For weekend aggressive driving, like the recent Julian Run (see recent Thread on that), the S6 will keep up with the stock RS6 in almost all situations, given equally competent drivers. Of course, the new RS6 when it arrives Stateside as a sedan will smoke it, but that's apples to oranges.
Can't go wrong with either one. Great cars.