Both are the same Ruby, but you left out 'with'. Because I'm still not finished with my assignment, I can't go to a movie. Since I'm not finished with my assignment, I can't go to a movie.

In common usage, "since" and "because" are both used in you example sentence... Some picky grammarians (and newspaper editors) only want people to use "since" when they are referring to the passage of time... Ex. - Since yesterday, I have not been hungry. Since the last time we met, I have have learned a lot. Since I was a child I have always loved to eat ice cream. ...however, if you're talking about cause-and-effect, you should use "because": Because I'm taking medicine, I'm not very hungry. Because I've been studying a lot, I have learned a lot of new information. Because I like sweet things, I like to eat ice cream. BUT...this is maybe too picky...most people will not notice or care if you use either "because" or "since" in your example.