I would say that if you don't have experience as a teacher, then a 4 week, 120 hour intensive TEFL course would be greatly benificial because they all have hands-on teaching experience as part of the course.
Having myself gone through a TEFL course, even though I had been working as a corporate trainer, I would have been ill-prepared to teach an English class. Even after the course - I'm sure you can appreciate the difference between the idealic conditions presented to a teacher-in-training for a class to teach, versus real-world teaching.
There's truly nothing to prepare you for that.
If you can find someplace in your community to volunteer as a an ESL teacher - and you're still wanting to save some cash, then I'd say go for it with the online course.
However, if that's not a possibility then I'd say go for the TEFL course - there are over 350 schools listed on this site alone - if there's a school in the country of destination that you want to teach in, then why not get certified there and get a jump on acclimating to the culture, creating a network, etc...?
Anyway, I hope that helps.