Which one is better, OpenAI or Open Assistant?
OpenAI is a company while OpenAssistant is a language model. So that would be comparing apples to the apple salesman.

Assuming you mean to compare OpenAssistant to OpenAI's ChatGPT. They are both language models who can reply to the same questions differently. So, they are just different and there isn't a clear better one in the traditional sense.

That being said, it's OpenAssistant 😉
Compare an apple to an orange in terms of sweetness.