- Not sure if I'm a good fit for the program
- No interest in the assigned advisor's research. Will need to find a new advisor
- Heavy course load
Are you straight out of UG or have you worked for a while ? Do you have prior research experience in your field of interest? It might be better to go through some course-work if the answers to the above questions are no. You can also choose your advisor while you do some of these courses.
However, if you already share a very good rapport with your School A prof., and you are pretty sure about your field/sub-field, then it would be more sensible to go to school A. Have you worked with him before ? Also, if School B doesn't have faculty active in your research area (?), then School A would be the choice.