[Actually, neither. What is the most important factor in terms of coral growth is a bulb's photosynthetic photon flux density (PPFD). This is measurement which indicates the intensity of a bulb within the spectrum of light used by corals for photosynthesis. This is commonly referred to photosynthetically active radiation(PAR).
In the general sense, the greater wattage a bulb, the greater it's PPFD/PAR. However this is not necessarily the case. There are several 250W bulbs out there with greater PPFD than 400W ones.
In terms of degrees Kelvin, this has pretty much just evolved into a marketing term used to indicate the "color" of a bulb as seen by the human eye. It bears no relation to intensity at all, and hence means very little in terms of meeting your coral's requirements for growth.
If you search the archives on reefcentral or reefs.org for any of the work done by Dr Sanjay Joshi, you can find the PPFD values for most of the MH bulbs in the hobby. His papers are easy to follow, and really lay the foundation for a great understanding about aquarium lighting.]
