I wouldn't say that one is better than the other. It all depends on the way you conduct your analysis and discussions in your work. This choice is part of your methodological cut, so it must be integrated to the other parts of your work. I would say that the interpretation provided to the data are even more important than the representation itself, be it a graph, be it a tree. We can continue our conversation, but I hope my main point here be clear.

I am not quite sure how this question makes sense. Obviously, a 'tree' is a type of graphic. Do you mean 'which graph is most suitable for representing different kinds of information?'

A pie-chart, for example, is very useful to show portionality, tree graphs highlight relationships, flow-charts indicate processes etc.

The choice between a graph and a tree depends on your methodology and data analysis.