They do sound very similar, apart from Page resolving the sequence at the end, and the issue with Led Zep is they have "borrowed" in countless other songs...however, that doesn´t mean the writer of Spirit deserves the credit. If it is such a common chord sequence or melodic line, then Zep should be able to come up with a number of very similar examples to prove their point shouldn´t they?
You can´t copyright a chord sequence, which is obviously fair, but melodies are a different thing. Stairway and Spirit both have similar melodies in the sequence. However, are they common or not? Where are the countless other examples?
I think that has to be the argument. I didn´t hear much of a difference between the two and as I have never heard any similar examples it sounds like it was borrowed to me. Page did it a lot and he has history, however great a musician he was in his own right.
Saying that, it would worry me if Spirit won this. It´s not a precedent most musicians and writers are going to feel comfortable with.