I too am a fan of the GS300 but I didn't consider it at the time I bought the Q because back then it was a good 5-7K more and being that it did not have a V8 I couldn't justify the price. I do think that the GS300 is a handsome design (Gugario). In Japan you can (could) get it with a V8! Anyhow I would drive one first. I spoke with another person who had originally looked at the GS300 but ended up with the Q because he said power wise the GS300 "was a dog." I would be curious to see what your findings are. The GS300 is a more compact size and would be easier to park and manuver through tight spots. Reliability wise I know that they are bullet proof. They use the same inline six as the Toyota Supra (N/A).
The A6/S6 is a beautiful car but... it's German! If you can live with mechanical and electrical gremilins then go for it. But if you have become accustomed to the mostly worry free aspects to Japanses car ownership, then I'd stay away.
Do you have a price range you'd like to stay within?
If you want to try something German try looking at an 500e (W124).
http://www.mercedesshop.com/500e.htmThis car kicks ***!!! 322hp & and a Porsche suspension. Need I say more!
I myself wanted to try something different and went with a 400e (W124).
http://www.mercedesshop.com/400e.htmI do like the car and it has grown on me. But if I was to do it over I think I'd seek out a really clean 1st Gen Q! I did a comparison between the Q and the 400e some tiime ago in this forum (do a search). If you'd like any more info about the 400e let me know. If you are interested in any MBZs check out this forum:
http://www.mercedesshop.com/shopforum/index.php3
Lots of good info here.