Assistants API and Chat Completions API with Same Model

My app create educational courses for users with the use of function calling, and interacts with users when they have questions or need any help.

I’m a bit unsure whether to use the Assistants API or the chat Completions API.

When I’m creating one of the courses, I do it in seperate calls to the API, but I’m not adding previous messages to history. This seems to be working well enough as I give the AI key topics that it has talked about already in previous parts of the course. I can do this with the Completions API.

However, I feel it might be wise to use the Assistants API when interacting with users.

Is that a good idea? And is it possible to use the Completions API and the Assistants API with the same fine-tuned model?