Gandalf the white is more powerful because the whole story is about the fall. The fall from grace, wisdon, loyalty...anything. Anything that sets the creator apart from the sub-creators, ie-Eru from Valar, Maiar, elves, men, etc. All the main characters experience a fall of some kind, some repent some don't, thus we observe the duality of good vs evil as polar expressions of divinity. This is a recurring theme. As Gandalf's "fall" is based on a sacrifice to the higher good (Eru), his re-embodiment is a divine blessing for the benefit of the sub-creators. He cannot conquer, only inspire. He cannot force anything to his will but he can use force to preserve the will of others. He is, in a sence, Eru! The Valar and Maiar are "the offspring of the thought of illuvitar". Which we can take to mean, aspects of the divine self. Gandalf the White is God, embodied and Sauron is mearly dancing to the divine tune. Gandalf's greatest power is his ability to know how Sauron will react. If you knew all the moves your opponent would make beforehand, could you really lose at chess? No. The overall message is the choice of the sub-creators to follow the wisdom of the creator. Frodo is the hero who trusted, had faith in, Gandalf. As is Sam, Aragorn, Theoden and even Butterbur. Who trusted Sauron?