From my personal experience …
The MA process and university teaching experience helped me as I transitioned back home. No questions regarding my degree, AFAIK.
I’ve posted about completing an MA in Taiwan. Scott’s posts on his PhD pursuit are likely more useful to you, in your position. I would encourage you to find a competent adviser - I had trouble with “highly competent” people agreeing to take me on, and then not returning phone calls or emails (I was advised to find them again and basically hound them into following though on the deal - found a better option soon after).
Teaching with an MA is still possible - I know that a few schools are looking for new blood rather consistently, which is a sign in itself. Overall, you wind up with significantly more responsibility as compared to bushiban teaching for less money. That said, you do get 3 months off per year, and are paid a 1.5 month bonus (in your second year).
The teaching itself varies - I have a business background, so my 1st/2nd year classes were full of commerce students, which was good as at least some of them realized the benefits that could be attained by competence in English speaking/writing. Colleagues with science/engineering students spoke often of trying to get blood from a stone. With only an MA, you are extremely unlikely to get any student who actually want to study English as a major - those classes generally go to the local PhD instructors, whether or not they can communicate in English at all (some are great, others would make you shudder). That’s just the way it is.
Where I taught, I had the freedom to give the grades I wanted - students who did nothing failed. When they complained to the university, I sometimes had to deal with it, but I was never forced to change a grade. (I applied at other universities that subsequently told me I would not be able to fail students under any circumstances - a non-starter from my position.) In the end, I was getting pretty good students in my classes, as I had formed a reputation for holding interesting classes, for being consistent, and for being fair - do what I ask and receive the grade you earned.
A PhD can open doors for you - even starting a PhD program, as Turton mentions. I was offered a much better position at more than one university if I was willing to start a PhD program. Be wary of any university that states they can offer Associate professorships without the PhD (or starting the program) - I have a friend who got screwed that way. He’s now working on his PhD though.
Hope there is something useful for you.