Obviously a master student with some beneficial research efforts instead of an extra subject will boost up his/her confidence in future, but students sometimes want to do masters with extra subject due some reasons specific to them. I'm asking for some motivating and appealing answers so that a master student should give extra respect to their research tasks, which will route some new directions for the humanity around the globe.
I would choose extra research over an extra course. The experience you will get out of a research can be invaluable as in analysis, logic, critical thinking, writing and submitting a paper and so on. For me, if you like a course and can't get enroll in it, you can always buy a book or get some notes and read it on your own time. It is much harder to do research in your own as an MS student. Of course the aforementioned points can vary based on your field. From a civil engineering PhD student point of view, research weighs more than courses. PS, you can always attend courses as a listener (for no credits) or take it if you do a PhD.