When you see Steyn's strike rate it would appear like a obvious choice on that one but I think Donald was better, and unfortunately we missed a fair bit of Donald's prime in International cricket due to things out of his hand.
Both had/have such lovely action, but Donald's run up was more awesome than Steyn's.
Despite the fact Steyn is such a provocative figure and he generally strikes fear into opposing batsmen, I generally think Donald was on another level.