The 4020 vs 806 would be the better comparison.  4020 for all around work.  If just to pull a plow all day then the 806.  The JD A and G were competitors to the M.  M for all around work including haying and loader work.  Hand clutchs don't work very good with loaders.  The JDs if just to plow all day.  Never been around corn planters or the rifles.  Definitely take a 7720 over the L3.  L3 might have been nicer to run but not as reliable.  Plus you don't have to worry about the swivel auger falling off the combine.  I like the Steiger better but the Versatile was also a good tractor.  And about the comment about farmers not wanting 2 cylinders after WW II.  JD actually caught up to IH with the 2 bangers and even outsold them for a year or two before the New Generation tractors.