From all the reading, research, debatimg, 3500K is the more ideal spectrum since it's closer to actual daylight.. If you can get a couple different ones across the spectrum, then you can optimize your light.. I run the HPS, 2 AutoCOBs (1-3500K, 1-4100K), and a 6-COB panel, all 3500K.. when I can afford to out together a UVA/B light, then I may ditch the HPS, although I kinda need the heat from the HPS in the winter when my basement can get down into the low 50F's.. I have heat mats, but try to avoid using them except for when lights are off..