If you play as an Amateur Detective, you'll get some easier games, puzzle hints on Nancy's phone, and a more complete task list.

If you play as a Master Detective, some of the games are harder, there are no hints, and the task list only has a few general items on it (like "Meet everyone" rather than listing all their names).

I like to play as an amateur the first time through so I can concentrate on the story, then play again as a master. But any way you do it is fine.