I was about to buy the A5, but made the mistake of sitting in the S. The seat comfort, adaptive suspension, and virtual cockpit sold me on it.
The "flat" seats in some of the A versions of the Audis look awful, whereas the S seats are so much better looking, not to mention more supportive (assuming they fit your body style). The seats and the flat bottom wheel are two of the big things that have me lusting after the S3.