First of all, it is not really accurate to portray these things as mutually exclusive possibilities. Instead, we can certainly say that sociology should be both of these things.
Let us say, for example, that we want to reform society through improving education. You would need to use dispassionate sociological research to figure out how to do this. You would have to, for example, try to determine how much difference good teachers make for poorer students. You would have to determine how much difference the students’ home lives make. You have to know all of these things in order to know how to achieve the reforms that you want. If you want to reform society, you need to have actual data that comes from dispassionate research.
If we do have to choose between these, however, I would say that it is better to be dispassionate than to be partisan. If you conduct social sciences research in order to try to prove a preconceived idea, you will generally get bad results. You will tend to prove what you already believed beforehand. This is no help to anyone. Only dispassionate research gets true facts that can be useful to anyone.