I realise this thread is probably well dead by now, but after sifting through the opinions of the scholarly LOTRelites, it seems to me that the situation could be explained thus:
Putting aside the fact that Jackson totally made up Gandalf's staff being broken anyway, if it HAD happened (which makes the whole thing idle speculation, but whatever...) then it wouldn't have meant that Gandalf was lesser than the WK, because, as has been repeatedly said, his power was 'veiled'. If we assume that there is a greater power responsible for setting limits to the power its minions can use on middle earth, then it stands to reason that Gandalf might not be able to defeat the WT, since he is PROHIBITED from the full use of his power.
Since a staff is just a tool, and not an integral 'component' of a wizard, it doesn't diminish the full potential of Gandalf's power that it be broken, even if Gandalf were not allowed to prevent its being broken, because he has to deal with the limits set on him from 'above'.
Extrapolating from what others have said, Gandalf was sent back slightly more 'unveiled' than before, as Gandalf the White. Whether or not his mortal form perished in the literal sense is neither here nor there, since 'Gandalf' is quintessentially the entity within. That being said, even if Gandalf had been killed, or could have been killed by the WK - which seems highly unlikely in any place - it wouldn't mean that he was fundamentally lesser than the witchking; it would just mean that he'd been screwed over by the Greater Power which sent him back to Middle Earth which hadn't let him utilise the full extent of his power.
Not to belabor the point (which I probably already have), but if you consider another fantasy world like Midkemia where the magician Pug is to all extents and purposes super-powerful, the same amount of magic goes on anyway, and Pug just seems to sit around doing nothing. That's because if the author put him into action there'd be no story, since he'd win all the battles in about three seconds.
Gandalf, on the other hand is limited by the Greater Power (read Tolkien) so that even though he's probably just as awesome as Pug, he is literally held back from his resources to a) maintain the balance of good and evil; and b) not ruin the story by letting him just kill all the baddies with one 'abracadabra'.
In summary Gandalf is awesome; he's a team player; he's got an awesome white robe (and various staffs) and is definitely my favorite character from the Trilogy. :-)