diction is word/phrase choice in a writing, and jargon is a set of terms that are used within a specialized group--for example, legal terms used within a law office would be considered jargon. not everyone knows what a docket is, or what it means to be subpoenaed. diction is simply the words a writer chooses when crafting a work.
they're similar in the way that they're both parts of language and they're both rhetorical strategies. jargon can make a person seem more professional, as they use the specialized language of their skill, and that ties into diction because specialized word choice can have a great effect on people.