Gandalf's Showdown with Saruman: What Happened?

[Earniel]

Gandalf, when first re-appearing to the remainder of the Fellowship, says that in a way he now IS Saruman, the way Saruman was supposed to be. So.. perhaps there can only be one? *cue Highlander music*

Perhaps the Istari gain strenght from how well they perform their duty? that would explain how Gandalf became stronger after his defeat of the Balrog, whereas Saruman continually seem to loses his original power to compel, influence and convince people. He cannot convince Gandalf to choose his side. From there on he starts to use fear and force in earnest. He doesn't rely on GrÃ*ma alone to gain control of Rohan, he is already amassing an army for the final shove.