The question is whether Gandalf could have defeated the WK or whether the WK could have defeated Gandalf.
On many threads on this site we've discussed the EE version, with much gnashing and grinding of teeth from most people.
One point I haven't put forward is that, in matters of black and white, whether one is stronger than the other does not mean the weakest cannot win. The WK was finally undone by a hobbit and a human. Both of far less power than Gandalf, but this is Fact.
So, to say well Gandalf is stronger than the WK because of whatever or vice versa, can almost be seen as imaterial. If the case of the strongest always winning over the weakest applies, then Middle-earth would have been a totally different place.
PS, why do you think Tolkien wrote the WK scene this way? Gandalf and the WK are about to clash, and then the Rohirrim arrive to save the day. To me, they did save Gandalf from a possible Mortal clash with the WK. Why on earth would Tolkien write it this way? The cock crows, the sounds of the horns are heard, and the WK realises his best laid plans are crumbling around him. That's why he shot off straight away in the book AND film. He HAD to marshall his forces and confront this new threat.
PPS, why is everyone getting het up about Gandalf's staff breaking? I'm going to raise a new thread in the books section shortly to kick off a discussion on this. IMO the Staff is a very Symbolic, but not neccesarily an important device for a Wizard. He had a production line of them in the movie anyway