There is a clear difference between the application of both the charts:
Burn Up Chart shows the amount of work completed. It also shows if the scope of the work (if we are talking about Sprint Burn Up charts) then it will show you when the scope of the Sprint/Iteration changed while the Iteration had already started. For example, if the product owner and the team decide to negotiate and remove 1 story and add 2 stories or the team decides to take up additional story in the current Sprint to work on, this "increase" or change in scope will be visible on a Burn Up chart. The straight line at the top of a burn up chart represent the total scope of the Sprint. When the scope if increased, this line shows an increase/spike. Whereas this change will not be visible on a Burn Down chart. This increase in scope may result a flat line for few days on a Burn Down chart indicating that the team didn't make progress for a certain amount of days (due to the addition of work to their Sprint Backlog) and will show dip in the curve almost at the end or a day or 2 before the Sprint ends.
Burn Down Chart show the amount of work remaining to be completed. Hence only Burn Up chart shows the increase/changes in scope.
