TESOL and TEFL are not certification; they are acronyms for types of teaching. I would agree with Clare that the CELTA is the most widely recognised certificate.
I would suggest you Google around a bit about TEFL International before going with them- there has been a lot of controversy about them. I have not had any dealings with them and cannot say for sure what is accurate, but there are many articles along the lines of this one:
The TEFL Blacklist: TEFL International