I have never owned a Chris King headset but from my research, they seem to be top notch "classic" headsets. I own a bike with a Cane Creek S-3 and I love it, so from experience, I know Cane Creek is great.
One important thing to consider before choosing sides is this little statement I retrieved from the Cane Creek website:
Is the technology in other threadless headsets fundamentally different from that of Cane Creek and AheadSet?
No. The AheadSet design holds a patent (U.S. patent 5095770) for threadless headset technology. All other threadless headset manufacturers license the AheadSet technology. Examples include: Chris King, Syncros, Race Face, Tioga, FSA, Ritchey, Dia-Tech and WTB.
I had previously assumed that Chris King is the way to go. Now I realize that both companies seem to offer outstanding products.