im actually a big fan of both. hopes first attempts were a bit flawed, but the bulk of thier offerings, especially the more recent ones are fantastic. i just serviced a modern hope last night for a friend and it shows no signs of wear or fatigue after 18 months of use on a dh bike. i like also the way that because the bearings are easily removable from the cups that time is saved servicing. it took me roughly 25mins from start to finish. king headsets are a pain to service properly, cups out of the frame etc. makes it an hours job to service.
i dont think that there is a performance difference between the two, certainly i cannot feel any difference but i can see the positives of using a hope with long travel forks.
oh, and iv just realised that all of my u.k. designed/made bikes have hope, and my u.s. made bikes have king. purely by accident.