The way we have approached this particular subject is by listing the criteria prospective EFL teachers should apply when evaluating TEFL and TESOL programs. You can find them in the Guide's chapter EFL Teacher Certification Programs
I haven't heard anything about Oxford Seminars one way or the other. I believe they are a very large and well-established training program.
My own bias would be to study at EFL training programs offered by university departments of education, as I think they would necessarily be the most academically sound, but this is not always logistically possible.
Best of luck with this.