These words have a slightly different meaning. A proposition is a statement which you assert as being true; a hypothesis is a claim which you decide is either true or false based on evidence that you present.
You might start with some propositions to enable you to set the scene of your study then introduce a hypothesis that you wish to test.
there is a research methodological difference here. A hypothesis is a statement about possible state of affairs, formulated in such a way that you can test through some research whether the hypothesis is true or not. A proposition does not need to have this latter part; it can be an assumption or starting point for some research, without the specific need for testing the proposition. In many cases the wording of a hypothesis and proposition is very similar; but I would suggest that if you really for testing something, that you should then use the term hypothesis.