Whose to say / who’s to say?

Whose to say / who’s to say? Which word do I use?
Whose to say / who’s to say?

Ian asks, “Dear Aprostrophe Queen, It’s very important to me that I use apostrophes correctly, but this one has got me twisted and I hope that you can help.

I have been asked a question by email, and my immediate response was that I haven’t a clue, so I was about to shrug with the above response. Seeing it written down for possibly the first time ever sent my mind tumbling.

If I said, ‘Whose to say?’, that would be asking ‘Who has the responsibility to answer?’

If I said, ‘Who’s to say?’ that would be asking ‘Who is going to answer?’.

Which is the normally accepted question behind the phrase, or are they both worthy as a non-committal response? Thank you for any help you can give.

Kind regards, Ian”

Question:  Whose to say / who’s to say?