"Ha ha" said the clown…"Hee Hee" says Gobby

Joker- Green Goblin

Legend has it that Steve Ditko, Spider-Man’s co-creator, intended that the true identity of the Green Goblin would have been a completely anonymous individual —this apparently is not true, as Ditko has said that he would have preferred that it be a Daily Bugle employee.

If the former scenario became status quo, the character would have even more resembled that of the Joker, a character whose origins are best left completely to the reader’s imagination. A compelling back-story appeared in Alan Moore’s Killing Joke, although presented with the caveat that the Joker couldn’t himself tell whether it was true.

As it is, the Goblin was shown to be Norman Osborn, the father of Peter Parker’s pal Harry: this reveal was Stan Lee’s preference and thus apparently the reason Ditko left the series and Marvel in 1966. Osborn most notoriously killed Gwen Stacy and thus ended the Silver Age of Comics. The character was killed off shortly thereafter, only to be revived in the 1990s. Recently, he’s been said to have fathered twins with the girl he would later kill and then to have been a government operative, two remarkably ridiculous plot devices that I can’t believe any truly professional editor would countenance.

Commonalities:
Both are murderous jackanapes, and each are central to their opponent’s mythos.

Differences:
The Goblin is super strong and bears a personal connection to Spider-Man, whereas the Joker is a pure psychopath and the id gone berserk; his personal history is irrelevant w/r/t a desire to inculcate chaos.

Alternate histories:

JO: Jack Napier, a driven and unscrupulous engineer and businessman, discovers an experimental formula developed by an employee. He recreates the serum, which explodes and quickly enhances his strength and exacerbates his antisocial tendencies. He takes on the identity of the Joker and develops a number of practical joke-themed devices to further his schemes. When he attempts to take over organized crime in his town, he is opposed by the Blue Beetle. But the Joker is defeated by the Beetle numerous times, which culminates in the murder of the Beetle’s girlfriend and his own seeming death. It has been revealed recently —and stupidly!— that the Joker survived and has been tormenting the Beetle from afar and now —also stupidly!— works for the U.S. government.

GG: The criminal lunatic known as the Green Goblin has alluded to conflicting identities and origins, but none have ever been confirmed. What is known is that the individual was deformed in an accident, leaving his skin green and his mind utterly unhinged. The Goblin has in particular bedeviled Captain America, with whom he is obsessed, for years, has murdered hundreds, and is feared by others in the criminal fraternity. He is not motivated by illicit gain and power as much as he is to create total and enduring mayhem.