I own(ed) both the 190E and the 500SEL, and the S-Class is a much, much better car. My SEL is also more reliable. The tranny lasted about 130K and a rebuilt from Mercedes cost about $2500. I don't have any roughness problems with mine. I think if you can work on an 190E, you can also work on the SEL. Parts will be more expensive but ...
By the way, I traded in my 86' 190E with only 16000 miles on it because something was also going bad. I still drive my 84' 500SEL with 167000 miles on it daily - a great car.