I strongly recommend a PhD in the sociology of education, it keeps the doors open for both fields and doubles your chances.
If you Google Raewyn Connell and Karl Maton you will see that both professors hold a PhD in pure sociology.
Karl teaches the sociology of education in the Sociology Faculty.
Raewyn teaches sociology in the Education Faculty.
Because both scholars are associated with the sociology of education, they can and do teach subjects and supervise student in both faculties.
I am passionate about this topic, as my supervisor writes in the Sociology of Education and I am doing my EdD in the sociology of education.
Example
http://media.wix.com/ugd/654734_7e3b5f6f19484c79a1af47891c9e689a.pdf