Having owned a 95 E320 for five years now, I can attest to their abilities and disabilities. As mentioned above, there is an infallible history of head gasket failures and deterioration of the engine wiring harness on these cars that is perfect..............they will all fail!! Just a matter of time.
My head gasket went out at 95,000 miles. First indication that I had a wiring harness problem was at about 100,000 miles. Since the replacement of the head gasket requires removal of the head...you might as well spend just a little more and do a total overhaul of the head while it is out of the car. Total cost for the valve job (and new head gasket) was about $1500 from an independent shop. Dealer charges could nearly double that cost if you take it to your dealer.
Replacement of the engine wiring harness is a DIY project, if you are unsuccessful in getting Mercedes to replace your harness under their good will policy. They did replace mine even though the car was 7 years old and had 150,000 miles on it. I paid for three hours labor, but Mercedes picked up the cost of the harness (about $600) and the balance of the labor.
Once you have these two items sorted out, the car is bullet proof. I love the car and feel that I have a 300,000 mile life in the vehicle.
There are very few comparisons between the S320 and the E320. As mentioned above, the only similarity is the engine. I am not certain if the S320 has suffered the same head gasket and wiring harness issues that the E320 has.