I Agree, might as well get a 600w HPS. But then there are other factor to consider for end of use cost if one opts for the HPS route. First and foremost is that they run intensly hotter, so dependant on where one lives, that person will have to consider extracting the heat more efficiently which could be as drastic as using some form of refrigerant cooling device (Air conditioner, aquarium chiller to chill revervoir water if doing hydroponics, etc etc), which at the end of the day chews thru a power bill. Another cost factor is the life of the HPS globe. When I used HPS lights, I would always change the globes after each grow, sometimes stretching it out to every second grow. At a rate of $50 (Minimum) per globe, the cost added up.
For me personally I like COB better than QB, and the the only reason I put COB before QB is that if one of my COB chips die, I can replace that COB chip for $15-$30 (aud), rather than having to buy a new QB. Though I will say that light distribution on a QB is unmatched compared to COB