If you decide to stay in TESOL for a long time you may consider taking an MA TESOL. This can be a good way of improving your career prospects. It depends on what kind of teaching you want to do.
For many non native English speakers, it will already be quite clear whether a masters will help or not. Your school might actually be encouraging you to take an MA TESOL, and offering a promotion or pay rise for doing so. A year in an English speaking country will also be seen as a big advantage, which is one reason why some countries do not accept an online master's degree. However, many distance MA TESOL degrees include a face to face component, usually in the form of a summer school.
You also have the option of taking an MA TESOL in the country in which you are teaching. Many countries now run English language TESOL programs. The choices will be smaller, but it could be an option. It will certainly be cheaper. Many of the programs run part-time, and the fees are usually much lower.
You will need to decide on the quality of the courses yourself. There are a few disadvantages of taking this path. For one, the MA may not be widely recognized around the world. Some countries have a preference for MA TESOL degrees taken in an English speaking country. Also, you would not have the opportunity to practice your English living abroad - if you are a non-native speaking teacher.
Here a teacher in Germany talks about her experience taking a distance MA TEFL
If you are a native English speaker, or simply not in the above position, it may not be so clear. Having an MA TESOL will allow you to apply for positions in universities teaching English. This is usually not possible without a tesol masters degree. It will also allow you to go on to take a PhD TESOL, and possibly begin a career in research. Many ESL jobs in the Middle East require an masters in TESOL or applied linguistics.
It will not usually get you any kind of pay rise, promotion or better type of job in a private language school - anywhere. There may be a few exceptions to this - but they really are exceptions.
There are many different names for very similar things, and sometimes similar sounding names for quite different courses. MA TESOL is equivalent to MA TEFL, or MA in teaching English. Look carefully at the course details to see if the modules offered suit you. I took an MEd TESOL, which is the same as the above, except that it is taught within a school of education, as opposed to a school of applied linguistics.
It's important to look beyond the acronyms used, and study the course outline in detail. Some masters courses are education based, some are more linguistically focussed. Often the latter are known as MA in Applied Linguistics. Choose whether your interest is more in education or applied linguistics.
Both of these would help you find work in a university abroad teaching English. However, a master's in TESOL, in other words, one that is education based, should give you a better foundation in teaching. If you are particularly interested in researching an area of applied linguistics that is not education based, then the applied linguistics option would obviously be a better preparation for a PhD in this area.
Some master's degrees are in linguistics (not applied linguistics). This is going in a very different direction, and would not be suitable for a career in TESOL.
MA TEFL courses focussing on education usually have modules on: teaching methodology, teacher training, materials design and evaluation, ESP, psychology of language learning, curriculum design, testing, issues in language education, and so on.
MA TEFL courses focussing on applied linguistics often have modules on: lexis, grammar, discourse analysis, corpus linguistics, sociolinguistics, translation etc. Of course there is some overlap between the two. Often it is just a matter of one course emphasizing one aspect more than the other.
Apart from improving your career prospects there are other reasons for taking a tesol masters degree. It can expose you to new ideas in teaching, and allow you to reflect on your own teaching, and look at the theory behind the fashions that have emerged in TESOL, and are reflected in the different approaches taken by different English course books around the world.
Be aware that some universities and some departments of education around the world do not accept online TESOL masters degrees. Some do, but not all. You will need to check for your own situation.
If you are in the process of taking your master's, or even if you are just at the planning stage, it could be useful to know that most countries will require you to legalize your degree certificate, once you have it. If you want to use your MA outside of the country in which you took it, this will apply to you.
The process involves red tape, and can be expensive. Without legalizing your degree certificate - and most likely your degree transcripts too - you won't be able to take up employment in a university abroad.
So if you take a masters, and you wish to use it outside of the country in which you took it, the degree certificate must be legalized. Visit the website of the embassy of the relevant country for more details. For example if you take your masters in the UK and you want to teach at a university in Korea, you must contact the Korean embassy in London for details.
For a UK citizen there are 3 steps to the process. First you must have your degree notarized in the UK [The British Council sometimes offers this service if you are living abroad. Call them to check, as this will save you time and money]. Second you will need to have your degree stamped by the FCO in London. Third you will need it stamped by the embassy of the country in which you wish to teach English.
I enjoyed taking my MEd TESOL, but it was hard work. It involved a lot of reading every day, which was time consuming - but also very interesting. With more and more English teachers taking an MA, it it becoming more common for universities to ask for a PhD TESOL