I read somewhere -- on WetWebMedia I believe, but I can't swear to that, so take this comment fwiw -- I read that MH bulbs decline in their Kelvin values over the lifetime of the bulb. So, by that logic, a 20k bulb should have a longer usable lifespan. I would be facinated to see a test done, using whatever instrument it is that is used to measure such things (lux meter? PAR meter?). Light output could be measured in situ each month, so we could know the real story. ..Karl