The comparison that makes sense is when there's an open race for the nomination. If there's an incumbent Republican President, that's who's most likely to win the caucus and the nomination.
Six open races since 1976. Two caucus winners went on to win the nomination (Dole 1996, Bush 2000). Four didn't (Bush 1980, Dole 1988, Huckabee 2008, Santorum, 2012).
Of course, Dole was from a neighboring state and that probably helped him win Iowa, but it doesn't look like the state is that great a predictor of who will win the nomination.