I am sure I read that (think 2005 model year) Mercedes had made some improvements on the c-class seats.That could be an issue. I usually take high speed one hour journeys in my C240 and the seats are perfect, but the times I spend three hours at a go in the seat, I need to readjust or get out and stretch. I am into long distance running and deadlifting and often have lower back and leg soreness.
I would GUESS that an S-class would be MUCH more comfortable on a 500 mile romp.

As far as leather seating surfaces, on base 2002 SEDANS, the outer non perforated fabric which takes the abrasive wear of sliding into position is not leather, while the inner perforated seating surface is leather.