Online Certificates are easy and cheap, if you just want a piece of paper that certifies you completed a teaching course then this is the best. Some employers dont care, they just want this piece of paper. It is a good way to get some book knowledge of ESL for a low price. But, experience is very important, these courses won't offer you any experience, you have to earn that on your own.

CELTA and four week courses are expensive, but if you are inexperienced, these courses are extremely useful. CELTA will be very useful if you want to work in Europe, or for a big company like International House or English First. Although, these are the best courses, the time and money is not always so easy for some people to sacrifice.

The combo courses are good, make sure you get your money's worth. Don't spend alot of money on something that is not accredited.

It all depends what you want and what your employer requires you to have.
You can always do any of these courses later in time, you don't have to spend all of your money for one at once. Take your time, get what you need, try teaching for some time, and if you like it, then spend the big money for something that will be a good career boost.