How does 126 #3 compare to 123 "S"?

[dbgrate]

My indie mechanic has suggested that I drive around in "3" in my '87 300SDL,when I'm cruising around the city,up to around 40 mph...use "D" for higher speeds and rpms.Can I take this suggestion to mean it's not a bad idea to use "S" while cruising around in an '83 240D??Are the "'3" in the 126 and the"S" in the 123 comparable??TIA for the usual variety of good advice!