I believe you are wrong. The 94 S420 does not have power rear seats, heated rear seat, SLS, charcoal A/C filter, and ASR that all add weight and came standard on a 94 500 along with a heavier engine - larger bore and longer stroke for larger pistons and longer connecting rods times 8. There's definitely a weight difference.
There's no weight difference between the 4.2 and 5L M119's. Trial and error on the spring pads. Start with the stock pads and if the result doesn't suit your taste, then adjust accordingly.