What will help a master student more a Research or One extra subject?
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.