pay no mind to ockmed, hes rather useless, he's also the guy that LOVES purple-up(which we all know does NOTHING!)
the kelvin rating doesnt affect the brightness, its just the color range and spectrum it gives off. 6500k is daylight(kinda yellowish white)
10000k is amost a pure white color to a cool white
12000k is white with a bit of a blueish hue
20000k is pure blue
Algae loves the lower kelvin spectrum,
best coral growth is around the 10000k-15000k,
Slow coral growth is the 20000k
you'd probably be better off comparing lumens to lumens as opposed to watts