There are obvious advantages of studying online. You can study at your own pace and continue a part-time or even full-time job at the same time. They are also usually cheaper than in-person classes, and affordability is something that is very important to many TEFL teachers especially as they need to save a little cash to get settled into their new environment. You don’t need to pay expensive additional housing costs or personal expenses that you would if you were studying in-person away from your home. Many schools and language centres in Asia and the Middle East recruit from overseas, so you can often be interviewed online using Skype or similar.
The advantages of enrolling on a course in-person are especially clear if you are thinking of studying in the country in which you wish to teach after completing the TEFL course. You will have experienced the culture and begun to get to know the new city or town and perhaps made some contacts at local schools. This is especially important if you are hoping to work in Europe or Latin America, where many teachers need to be in the country for a local interview.
Another great thing about studying in-person is the social aspect. You will make friends with the other soon-to-be teachers, all of whom will be in the same situation as you. Most in-person TEFL courses have a maximum of 12 students in them. Keep in touch with these people after the courses so that you can share tips and advice about teaching in different countries and at different schools.
Online classes have a social aspect too, as there are online lectures and webinars (online seminars) which encourage interaction between students. You also have to complete your practice teaching modules which normally consist of a minimum of 20 hours. This is usually done in your local community and is generally arranged by you.
If you are studying in-person back in your home country, it may be possible to enrol on a course in your own town or city or alternatively stay with friends or relatives rent-free. This would go a long way to keeping your costs down.
Online TEFL courses usually take at least twice as long to complete as the intensive 4 weeks of most in-person TEFL courses. Which suits you the best really depends on whether or not you need to continue working alongside your studies and just how soon you want to start your new career.