In the cold climates. Rust is an issue with both cars. I've seen some sub-frame replacement on both cars which here in North America. It is just unheard of. We have the issues of heat-paint break down-rubber body seals cracking-etc..

I have both cars-Two W140's one W210(wagon). If I had to get rid of two cars. I'd keep my 1996 S320. Easy to work on-lots of engine space. No hydraulic shocks or heated seats, yet like all W140s it just has more parts to fail than W210. If you do not like to tinker, they can burden a person. For me the pay-off is the ride. The fuel economy is a little better than my W210. W140 has a higher interval of maintenance compared to any other car. Most are just run down-Antennas do not work-close assist pumps leaking-crack or torn leather seats...

Since you rebuild transmissions, 96 was the last Hydraulic transmission on the S320, 1995 for the V8 & V12 models. After those years,the first generation 722.6's came into play 1996 and 1997 had many revisions. It had so many revisions that 96/97 will not work 98/99 car. K2 bearing is a big issue on the 722.6 from 96 to early 99, and when the bearing goes one has little time before the planetary gears destroy everything, and sadly the shaft has to be replaced because Mercedes updated the shaft it will not mate to the other shaft(because of new bearing size), so that shaft has to be purchased too. The price for each shaft(front and rear) at wholesale Mercedes prices is about $750USD(that is If you have a connection with a wholesale MERCEDES dealer). This is a problem for the W210 of 96-99 too.

Early W140's are more stylish, and are more elegant, but evaporator and engine wiring issues plag the early years. For some reason the early W140's we hard on ignition issues. The later W140s with no distributors just work all the time with no issues...

The W210 has the rust issue with spring perch on the non 4matic version. The spring support will just break. If you want something practical W210 is the best choice. Just get 2000 - 2002 with the second generation 722.6. the car will run like a top

I like big cars, so my opinion is just skewed. I like to tinker, and the ride on W140 is just worth the extra effort.

Either car is fine...

Martin