I know this isn’t a new thought, per se, but most people pondering this like to pit the two villains against each other. What I’m saying is that they are basically the same character: both are the yang to their opposite’s yin; both have an affinity for purple and green …
And, since Joker appeared in 1940 (in Batman #1) and Green Goblin first appeared in 1964 (in Spider-Man #14), we can see who influenced who.
I guess what I’m saying is that possibly Stan Lee’s genius lies in his creative marketing, not his creative characterization.
Kind of reminds me of that whole Batman/Iron Man revelation.