Both are similar as you've recognized and both are nouns. I believe the difference is in their application. Diligence may be considered more about an attitude, the fact that someone considers all the details to leave nothing out of their work, to be as complete and proper as possible. Effort is usually something we can see, more behavioural; the time someone puts into something, the calls they make and research they do, and the conferring with others maybe, in order to get something done and done well.