There are different reasons to take on or another test. By the way there are so many of them!
Some, like CAE,FCE etc are types of Tests,that show weather you up to a certain level. Some use another system and show at the end how well and not good you know and can use language. The Criteria is set in accordance on the Common European Framework and alike whichever language you are testing.
IELTS is one of the most popular exxam. It is long and thorough, accepted by most schools ,Universities, Colleges etc But it is valid for 2 years.
CAE FCE PET and alike can be used any time as proof of your English.
Then it is worth to decide where ?
IN Americas you find easier use of TOFEL, In Australia, The UK and other Commonwealth countries Cambridge tests.
For work, I would take recommend to think about BEC (Cambridge Business English Certificate).
There are also Academic tests... 
What to choose?
I would say - make your OWN mind! Read ARTICLES , read sites of the examination bodies , British Council and write to them if you need to find out more.