I own both. My KL5 handles heat better than the KL6. The KL6 heat sink, according to CM, is to thin to quickly move heat away from the emiter, which causes output to drop fairly fast after light is turned on. My recently purchased, new KL5's holds its output until the light drops out of regulation due to depleated battery (s).
Even with diminished output my KL6 out throws my KL5, and the KL5 has an overachiever lux V emiter. I would never sell my KL5, but the KL6 is another story.
Bill