There's a good blog thats started reviewing the online courses critically, wish I could find it again. Stands in stark contrast to the likely-paid-for fluff at teflcoursereview.com, tefl.com etc etc. Not a fancy page, literally just some longterm EFL dude taking these courses and reviewing them in his spare time. Long story short, there's no official accrediting body for these courses, so any that advertise that you should view with suspicion. Backed by a reputable university (not an online paper mill) is perhaps a better thing to keep an eye out for.
The two big "highly-recommended" online ones you'll see on most review sites both regularly troll forums with "3rd party" advertising, so whilst not to say they're not good, take any advice that's not from someone trusted with a pinch of salt. I took one of them and the course notes were I'd wager 75% padding and weaselwords and wouldn't even pass a MS Word spelling and grammar check leaving you paying strictly for (the more professionally presented) assessments.