I'd say 'eɪ' is used when you are speaking more slowly, or when you want to be more clear. 'ə' is easier to say, and I would say is more casual, and so it is used in fast conversation :) However, really you can use both whenever you like, there is no real correct answer.

Both are correct. You can pronounce 'a' either way but I think 'eɪ' is more casually.