Trying to compare the (C6) S6 vs the (C5) RS6 is like trying to say which Playboy Playmate is better, Ms. October or Ms. November?
As far as the 2003 RS6 looking "dated"... I think that really only applies to people who own Audis. Honestly, until I bought my first RS6 I never paid any attention to Audis (Volvos as well, still don't). They all sort of looked the same to me with the exeption of the TT. I think that 90% of the population does not have a clue as to what an S6 or RS6 is. Maybe more like 99%. With updated LED tail lights on the RS6 the car looks very similar to the C6. The big grill being the exception. No one seems to know what year my RS6 is. I mean no one. Shock and surprise when I tell my clients "Oh it's an older 2003 Audi, no big deal..." It is one of those cars that if they look close enough they see the big brakes and the wheel arches, not quite sure what it is but they know if is something special.
That said, I too almost purchased an S6 recently that was at a local Audi dealer. They had taken $15K off the list price. I liked the interior and upgraded electronics, etc. Recaros were also nice. While it drove nice it was not anywhere close to producing the power of the RS6, esp. with the Revo upgrade I'm running. To say it was slow is not correct. It was plenty fast to get me thrown in jail (twice) . Something about the boost on the RS6 that puts me in that "happy place". For now I'm waiting for the C6 RS6 Sedan to arrive (still have a deposit) and if they don't bring it over then I'll seriously have a tough time finding a suitable replacement in a few years. I've even considered buying another RS6 with low miles and storing it until this one has racked up 100K miles. I just won't tell my wife...