I am researching teacher training programs for teaching English overseas. There seem to be huge variety -- with different acronyms, TEFL TESOL etc. I suppose I wonder which might be the best investment for me, if people have thoughts.

To teach English overseas the general usual requirements are a degree and a TEFL certificate. The TEFL cert is sometimes known as the TESOL or TESL cert. To all intents and purposes there is little difference between these types of cert and most employers will not be concerned which acronym is on the certificate.