Tesol is cheap and easy to pass but Celta puts you in a good place if you want to think further than "just getting a job to teach" for one year if you see it as a long term career. Celta is harder and expensive in terms of assignments and exams, but you learn a lot and is preferred over tesol if they would want someone with substance. But if you just want to swing it and have a visa to stay in Thailand and it's just a temp experience, then tesol would be enough for Thailand.
At my language school, two farang young guys were denied ed visa (got a void stamp in their passport) for 2nd year even though 2nd year is to learn Chinese. There are a lot of different things to look at that could be a reason for them to reject your visa. It doesn't mean if you learn Chinese the first time it's a guarantee to get the visa. But if you trust your school and their connections, and their experiences with handling visas, try to listen to them. 5,000b additional is not a lot to study Chinese. My school asks for 20,000b more to study Chinese. May I ask which school you go to? I prefer to study Chinese as that language is more universally used in the world than Thai.