Is it worth getting a TESOL diploma as opposed to just a cer...

I would be curious to know what school you are talking about. It sounds like the one I went to "Global TESOL College" (hmph to the college part). I paid for the certificate and did "specializations". I chose business, for the reasons you mention, but having no experience of knowledge of business, had to drop it.

I honestly don´t think it makes any difference, that you just need a certificate and your other experience teaching, or in business is more valuable.

I have worked as an ESL teacher for 6 yrs and traveled in 64 nations and it is my humble opinion that it’s a BIG waste of money and time. All the schools I have taught at could have cared less. Just be funny and joke around and the student they will love you. You are as good as gold, especially in China. I find the student Like you because they like you not because of a piece of paper. Save your money and travel around and have fun……. Good luck and god speed.

In my opinion, the answer is "yes" if you´re looking for jobs in Taiwan, Japan or South Korea, particularly in the private sector.