How do I determine the appropriate fixed variable task type for a project?

Project allows you to set the task type to either fixed units, fixed work, or fixed duration.  Fixed duration I’ve typically set for like summary tasks that represent a sprint or a meeting type task.  When would be ideal project scenarios to use Fixed Work or Fixed Unit task types.   My opinion is to set the default task type to fixed work because people may not work on a task the same amount  such as one day they may be able to work a half day on a task putting them at 50% units, another day maybe only a couple hours/25%.   To me the most variable that would most likely stay fixed is Total Work.  

So if I set my task types to fixed work but I update tasks based on being given a % Complete, would that affect the fixed work variable at all compared to using day fixed units?