It seems to me that your question has a lot to do with the bulb and ballast combination from the standpoints of intensity and color.
Although light diffuses and loses its intensity much more in water than in air, the distance (24 inches vs. 24 minus a few inches) is not significant enough to make much difference in intensity.
Although 20K is less intense than 14K in terms of PAR, I have not experienced a really big difference in the growth of corals in my tank. So it seems to me that it is just a matter of preference. Some people don't like the stronger blue of 20K. However, the blue color lessens in about a month or two. My 20K XM bulbs looked like 14K Phoenix bulbs in a few months. When I switched from 20K XM (XDE actually) to 14K Phoenix, I did not see much difference except for my blue eye cardinalfish which lost its intense blue glow under the new 14K.
Tomoko
