The difference is that you are picking on a particular nationality, while "retarded" is a generic term meaning "mentally under-developed" which, while certainly not a term of endearment, may apply to any human.
In the first case, you imply that Jews are stingy. In the second case, you don't imply anything about retards other than the normal definition of the word.
Do you then continue the joke? "stop being such a Jew! Give me a chip!"
Are you saying it's fine to use that word because it's just a joke? It's not a serious insult?