My writeup at https://mbworld.org/forums/s-class-w220/124317-comparison-00-05-w220.html
responds to some parts of your question, but not all. I have fairly little experience with BMW except test drives - and while the cars (5-series and 7-series) handled well, I thought them a bit small, and liked the size and feel of the 2000 Audi A6 2.7T that I bought.
I truly enjoyed the 2.7T. The all-wheel drive (Quattro) gave tremendous road feel, and kept the car true in all kinds of snow and rain. It has a true command of the road that you just don't get any other way. The 2.7T suspension was firmer than the standard A6 (thought those were too soft) and close the the Sport suspension. The more recent standard A6 seems even softer than the '00. The A6 was well-executed, and gave me very few problems - except that radio reception was always weak (but the Bose audio system was very good - and the dual subs gave a tighter bass than I get from the large single sub in the my S500's Bose system).
However, when I drove my first W220 (an S500), I was sold, and traded the Audi - the luxury and feel of the S500 just simply blew the Audi away. However, my first S500 (a '00) was RWD. I really missed the AWD - I don't think all the non-slip gimmickry in RWD begins to equal the grip of a good AWD, no matter the road conditions - including dry. Looking to get back to AWD in 2005, I drove an '05 A8L and an '05 S500 4-Matic - and definitely preferred the look and feel of the S500. The drive systems were about equal. I bought the 4-Matic, and have it now.
As I mentioned in the link, reliability of the S-Class has increased, especially since the '03. I rate my '05 at least as good as my 2.7T was - it has been trouble free in terms of maintenance (but is still under 20K miles).