I think the fact that Olorin was "afraid" of Sauron doesn't necessarily reflect their individual potency. I think even the Valar were afraid of what Sauron was capable of. Olorin was chosen because of his humility -- a quality that suited the task at hand. We see what effect pride had in the example set by Curunir.
Gandalf was successful in his mission because, unlike Saruman, he never overestimated his power or underestimated that of the enemy. Saruman believed he could go punch-for-punch, so to speak, with Sauron, as is demonstrated by his unrestrained fascination and study of Sauron's dark ways and his use of the Palantir.
While Olorin may or may not have been greater than Sauron, we must consider that he was chosen specifically by Manwe for the task. This suggests to me that Manwe believed if anyone was capable of dealing with Sauron, it was Olorin. Counting them equals would not be a mistake, in my opinion.
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