I currently run a mk4 LS400, even though the car is in perfect condition, I fancied a change. As I cannot afford an LS430 I looked around for an alternative comparable vehicle. I did happen to find a Merc S430 at a good price. I managed to take this on a reasonable test drive, and therefore took the opportunity compare it with my car. First impressions of the Merc was of a much more noticably instrusive engine noise, the handling and balance were more sports like, and gave a great sense of security. Steering was heaver that I like, but had good feedback. Seats I found were very hard, but supportive. Bumps were felt much more than I was used to, this was added to by several squeaks and minor rattles from the cabin. Switch gear I found to be an ergonomic disaster, with everything seeming in the wrong place, the COMAND system is awful and would require a lot of manual reading to get the hang of it.
I have tried to be objective, but I was glad to get back to the serenity and comfort of my LS. I have now decided to sell my vehicle and pay the extra for an LS430. Did not try a Audi A8 though?.
Hope this is useful to anyone considering a change.