By your description it seems like the main or the principal track(s) of the conference are the symposium(s) and that the workshops are sided events. So, typically, workshops are smaller, more focused on a given subject or theme within a larger research field. The symposium is, typically, wider with several subjects being contemplated within a research field. For example, a conference in Software Engineering (wider) can have a workshop on Model-driven Software Engineering (specific).
So, usually, it is more easier to be accepted in a workshop, if and only if, your contribution is really focused on the subject of it. For the main track, it is typically more complicated because there should be clear contributions to the state of art in an area of study, with well-stated results and more detailed research.