I don't know that there is a person on earth that can give you an objective, informed answer to this question. It's about a subjective thing to begin with, then there's the fact that they played in different eras, and lastly there is more than one way to lead a team.
I think the only conclusion you can concretely arrive at is that they were both very good.
Unless you played with or coached one of, if not both, Clarke and Stevens, you probably have no idea who was a better leader. I have a feeling there aren't too many people who fulfill that category posting on this site.
They were both great leaders, from what we have seen and heard, but comparing leadership is absolutely pointless. Leadership is such an over-compared trait on HF. Good luck proving who was better.