yea.. this is a loaded question.. lol
there are alot of oppinons on this one..
its been said that high light coarl and critters do best with 6500K but its known to be very yellow (needing a good punch of 03
to hide it if you dont like it) 10,000K being better then the higher K
's.. the lower the Kelvin the more PAR/light the bulbs put out most generaly..
other then that its a matter of preferance with the color of the light.. alot of SW
aquarist really dig alot of blue.. Ive seen 20,000K and they have put out a new 50,000K bulb and people suplementing that with lots of 03
.. how fast the coarls grow compared to lower K
's I wouldnt know but man that would be blue if not purple.. LOL