Its all about personal preference. Your lower K bulbs give off more yellow, with the higher more blue. Having used XMs, Hamiltons, Reeflux, Coralvue, Coralife with everything from 6.5-14k. I would have to say that XM run a little more on the yellow side compared to other bulbs but they put out the most PAR of all them. Reeflux puts out a close second with a more blue tint to them. If you have been to TNT they use the 12k Reeflux on their main sps display. These are the bulbs that I am currently running also and have been happy with them. Also your lower K bulbs usually give you faster growth compared to the others. My favorite combination is Reeflux 12k bulbs with VHO super actinic giving you the best of both words, coloration closer to the 20k but with growth more along the lines of a 10k. Hope this helps a little.